He's a champion who requires skill, timing, and strategy to play while being a strong teamfighter and a good ranged bully top lane. People complained about him being weak when he was released because no one actually knew how to play him then and didn't have experience with his matchups. Now that people are learning how he should be built, how he fits into a team, and how his lane matchups work, he's recognized as being a strong and versatile pick. I'm not sure if he's worth banning (I've never had issues with Gnar's in my games, but I've not played against anything higher than Platinum Gnars, my usually champions can survive laning against ranged bullies pretty well, and I don't mind cowering in safety from lane bullies until I'm stronger than them), but he can be strong when played well.
If he's being picked or banned by pros, there are probably a lot of people just following the bandwagon and copying the pro players.
If you have trouble against Gnar, I would recommend picking 
Yorick and Tryndamere can withstand Gnar's harass early, beat his Mini form in trades easily, and get away from his Mega form. Cho'gath can easily harass down Gnar with his W and sustain through Gnar's poke. Irelia will just outtrade Gnar's mini form without much effoct after you level up W and get a Sheen assuming you play it well. Irelia, Maokai, Nasus, and Cho'gath can all susain through Gnar's damage and easily set up ganks with their mobility and CC, which should allow them and their jungler to easily burst Gnar.
