Portal 2 felt like a step backwards. I was very disappointed with it, and you don't hear anyone talk about it anymore. Looking at Steams stats, more people are playing Gary's mod today than Portal 2.
Portal 2 is a linear, puzzle-oriented single-player game with little to no replayability aisde from mods. Garry's Mod is very replayable since it's essentially a sandbox that fosters creativity. I'm pretty sure Garry's Mod had a consistently larger player base than the first Portal as well, particularly given the original's very brief length.
What specifically disappointed you in Portal 2?
On an unrelated note, it's great to see Fallout New Vegas almost always having thousands of players and remaining in the top 10 most played Steam games, despite the fact that it came out almost a year ago. I really hope that Obsidian gets to make more Fallout games.