Like others have said, compared to Gundam Breaker 3, this is terrible. Clunky mechanics, game freezes, and dull story. Either get Gundam Breaker 3 or buy some physical Gunpla to scratch that itch.
Ok, now that the short version is out of the way, let's get to something resembling a proper review. The whole point of the game is to collect parts, build your own gundam how you like, and play with it. You do this by knocking the parts off of enemies and collecting them. Spoilers? Ahead. Be ye warned.
Gameplay: Gameplay is fine, not good but its not so bad it's unplayable. But very clunky and repetitive. And seemingly designed to waste your time.
The core concept is that you fight other people's custom built gundam models. This is done in the "G-Cube Format" because there's three members on each team and you do battle, but not by directly attacking them, like you would expect, but by fulfilling quests, side-quests specifically is what other games would call them. Go kill X number of generic mob. Kill Y mob and collect it's part. Open Z many crates. And it just recycles those three over and over again. So the core gameplay is completing what would be optional sidequests in other games. Not fighting unique and interesting enemies.
I had issues with it recognizing an input or just being so slow with the reaction that I wasn't sure it had gone through which led to many instances of me trying to use one of the two basic ranged attack modes, press vs hold, and it either did nothing or did the wrong thing. Which is frustrating.
This issue carries over to the collection of parts. You can carry up to 5 parts that you picked up off of enemies. To do this, you have to move over them. Now you have to have not attacked nor been hit by an attack for about a second, otherwise you won't be able to pick up the parts. And then you take those parts to a crate that has spawned somewhere on the field, and you have to hold down the circle button for about 6 seconds to actually collect them, it does not matter if you are holding 1 part or 5, it is always 6 seconds, and then after a second or two the crate will disappear and several seconds later the crate will reappear in another random spot on the map. Hopefully, your input will be correctly recognized and you won't waste time holding the button down but it not recognizing it nor delaying its recognition so you let go just as the bar starts to move and have to start over. Your AI 'allies' will also be collecting parts, parts the AI has no use for since it's the AI and it's gunpla never change, and go deposit them in the crate. So if you had your eye on a particular part, hope the AI does not decide to take it. And if you want to deposit your parts and you see the AI heading for the crate, you should race after them at full speed because they won't wait for you before depositing their parts and sending the crate to who knows where.
When you get knocked down, you can't do anything to speed up the getting up process, not that I noticed at least, you're invulnerable while knocked down, but so are enemies. So what felt like should have been a brutal melee combo will sometimes end with the enemy getting hit by the first attack and then you being locked into the rest of the combo while others attack you because they're prone on the ground.
The Ex Skills of the parts you have equipped, your special attacks, buffs, or other abilities, are all locked behind leveling your inner frame during the level. The easiest way to do this is to find boxes around the level and attack them. Your gun will not lock onto them, but if you do manage to get a ranged attack in it'll break and you'll get about a fifth of a level. Sometimes enemies will give some xp when you kill them too, but that's not reliable. These levels are not shared across your team, so every crate the AI opens is basically just the game keeping you away from your abilities. Every inner frame level above 1 grants you access to an Ex-Skill, singular, at random. So that big aoe attack you actually want to use may be the first to unlock in which case, yay, or it may be the last to unlock in which case, suckage. So you either spend the first minute or two of any level reopening the same 5 boxes to try and get your desired abilities or you wade into the fray with nothing but your basic attacks and hope for good luck.
Game freezes and loading issues plague the game. About once a minute the game will freeze for 1 - 3 seconds and then continue. For no clear reason. And every time you go into the gunpla customization menu it freezes for about 5 seconds. Consistently. This isn't a freak accident that happens sometimes or often literally every single time. Accidentally backed out? You got to deal with the freeze again. At the start of every mission it shows your gunpla launching, except for when it doesn't load the model, which has happened more than once, and it will consistently take at least a second to load in when it does.
Level Design: The levels are ok and can be fairly expansive, but are repetitive.
All of the action will be happening in one corner of the map for about 90% of the level so it ends up squandering its real estate except for when it wants to send you to the ends of the earth to deposit parts.
There's also only a handful of maps, but well over half of the story missions take place on the same exact map. Where you repeat the same exact sidequests. Just with different characters saying things in Japanese in the background.
Campaign: The campaign, or story mode, is trash. Oh, sorry spoiler warning. You are a transfer student and you are the only one who will stand against the tyranny of the student council.
Conceptually, not terrible, not great, but I've seen better done with worse. Execution, horrible. You do a few missions that give you some background and set you up as the hero of Gupla High, I'm sorry, Gunbre High.
After those missions, three routes open up to you. One for each of the three starting waifus. There's the best friend character who apparently has been trying to stand up to the student council, but literally no one will ally with her. The one who looks like classic Japanese beauty, but is a little too into combat, who for some reason didn't want to fight the council before. And the nerdy glasses girl who is supposedly bad at combat but a great builder, but is willing to fight with you because the writers demand it. You do the exact same missions on each route. Your AI ally is different and there's slightly different dialogue before each mission, but the results are basically the same.
There's also dates, for some reason, so you know all of the missions are exactly the same, because they have to be, because that is what happens on that day in all three routes. After you beat one of the routes, more waifu routes become available.
Gunpla Building: The whole reason you're even interested in this game. Building a gundam and killing stuff with it. Aesthetically, you can build what you want, but it won't necessarily have the abilities you want. So your OC gunpla very well might not be possible to use. For me personally none of my favorite gunpla made it into this game.
All of your stats are tied to your inner frame and parts, neither of which can be improved. So if you really like the original GM you can build that, but it will never perform as well as a GM-II. On the one hand, that makes sense, the GM-II was the successor to the GM in canon, but this is Gunpla where your dreams should not just be dreams. Out there, somewhere, I'm sure somebody has figured out what the statistical best build is, and it's probably some hideous frankenstein or there's just an OP gundam built in. *shrug* I was only concerned with making something I liked.
You can freely combine 1/144 and 1/100 scale parts that you've collected which can be amusing, but makes no mechanical difference and while having a piece of either will unlock that gunpla's parts for purchase in the shop, or Library as it's called, one leg piece is not the same as the other which can be a little confusing and frustrating at times.
Your Ex-Skills are tied to your parts. So if you wanted to give your personal gunpla the active camouflage ability, you have to use a part that has it. And not every gunpla part will have the abilities you'd expect, yes most Gundam styled heads have vulcan guns, but no part of the DeathScythe Hell gunpla from Gundam Wing Endless Waltz has active camouflage like I was expecting.
The Builder Parts, accessories basically, that you can put on your gunpla provide passive stat mods, but don't grant access to any actual Ex-Skills. So if I build a basic GM and slap a ton of missile pods to it, because I love GMs and missiles, then I have a GM that has a ton of boosts to its vulcan guns Ex-Skill, but that's it. I cannot fire those missiles. Which is frustrating for me as someone who likes missile barrages. Or if I wanted to build a GM Sniper Custom II with active camo, the NITRO system, and like a a shoulder beam cape because that's my OC's gunpla, I have no way to actually make that with NGB.
The painting is fairly in depth. Every part can have its own swatches to pull from to paint predetermined sections. Main body, various armor layers, vents, etc. So that's pretty good.
Visuals: It's not like crazy high definition or anything, but it looks fine once everything loads in.
There are frame rate drops, but I didn't notice them too much. Not compared to it actually freezing for seconds at a time. Or taking time to load in at least.
The character art is good, but they're stills in front of a static background, just like a visual novel, so that's to be expected.
Audio: I'm not an audiophile nor do I understand Japanese, it sounded fine.
There were songs from a lot of the Gundam series, and you could pick some in the gundam build menu, but I'm not sure when they were played, might have been during missions? But I'm not sure.
It's a little annoying that we finally get a Gundam Breaker game released stateside and there's no English voice acting, but all things considered I'm not surprised. It was a little to no cost localization for a subpar game. Just like all of the SAO games.
Final Verdict: Do not buy. I get that they wanted to make it more accessible for people, making things simpler, but they made things too simple and shackled you with random chance. Neither the story nor the characters are engaging. The gameplay is frustrating, repetitive, slow, and dull. Gunpla customization is like having an arm shackled.
If you want to give it a spin for yourself then see about renting it. If you want to build a Gunpla, then spend the money on physical Gunpla. You can probably make exactly what you want IRL for the same money and less time than you would spend on this game and trying to get all the parts you want. Then go and play pretend Gunpla battle with your friends or kids, if you've got those. It'll be more fun.