What started off as a mod has evolved into a standalone game and one of the most highly immersive ones as well..........
Multiplayer, specifically Trouble in Terrorist Town, is a true jewel. Plenty of game modes are available, with lots of servers to play those on. TTT is the most popular gamemode, and is like a video-game version of Clue. DarkRP is another fun gametype as well. Sandbox is also prominent in Mp, though not as much. The Steam WorkShop has tons of mods to keep you entertained as well.
The community is still very active, which is great for a nearly 8 year old game. You will have very little trouble finding a good server. There are many truly brilliant community-made maps that are found in GMOD.