People don't really understand him very well. He is a little frail (being 3rd lightest character in game), his dodge roll is awful, and alot of his game takes more practice than some to pick up. Not to mention for some reason people don't seem to appreciate floaty characters.
However he has many plus sides I feel like are greatly under appreciated. He has 3 of the strongest moves in the game (Fully charged bucket and sometimes only a 2 charged bucket can kill off 0%, 9 hammer, and his Usmash can rival Lucas'), he has a chance to heal from his sideB (granted he also can hurt from it too), many moves have a lot of power, dtilt has superb range, and his down throw just slams them to the ground in front of you, setting up some crazy combos (dthrow to dsmash can guarantee a kill at certain percentages), has a great recovery, and he's really really hard to look at. Seriously, have you ever fought one? Or remember how it feels to fight one when you didn't main him? He is so annoying and distracting to watch him move that's borderline obnoxious! He's an unorthodox character with an oblong moveset, but that's why I love and play him. He will always go under appreciated and if he isn't then something has gone wrong.
Edit: Not saying he's top tier; he isn't. He is good, but not top tier mechanic-wise. You have to try to play the G&W to be good at G&W.