Gnar also has to give up significant amounts of survivability that's not completely made up for by his mega form to make his mini form actually viable, so much so that the guy who designed him has warned people away from items like Sunfire Cape or Randuin's Omen because those are all but useless on mini Gnar, who you'll spend 90% of the match as.
He's nerfed by his own kit. By the fact that you can't fully control when you're going to go mega, and once you do you either have to commit and get your use out of it or you're going down like a flaming little furball. Gnar is balanced by his lack of balance. By the fact that in a situation where you're low health and running from someone who CAN stick to you, and there are those who can, you might Mega Gnar right when you don't want to, when perhaps your ult isn't down, and you won't be able to get away. Or you might have just been trying to push a lane, go Mega, and the enemies wait until after it's worn off to jump onto you, when you can't retaliate well enough to overcome given how weak and squishy mega Gnar is.
Clearly you've lost a few matches against a Gnar, or perhaps just one very bad match more recently, but I can assure you as someone who woke up the day of his release eager to pay Riot more of my hardearned money and spent most of that day playing him, as someone who's knee jerk reaction anytime I have, or want, to go top lane is Gnar, that he doesn't need to be nerfed half so hard as you're making it out to be. Once he gets his snowball rolling he can be hard to stop, but that doesn't change a few key points.
Mega Gnar is easy to kite.
Mini Gnar is easy to jump on and kill.
You can't always be the Gnar you want to be.
CC will shut him down if used at the right time.
He still falls off late game.
Teemo still laughs at him.
My advice?
Darius' pull is as long as Gnar's AA range during the early stages of a game. As such a skilled Darius can shut down a Gnar pretty easy by whittling him down with his bleed passive so when Mega Gnar happens, Gnar isn't at a high enough health to go in on the Darius.
Irelia's passive helps her out in chases against, or from, a Gnar. Her equilibrium strike, the BoRK you most likely have on her, and a Triforce keeps him in range if he tries to run, and eats him alive if he tries to stay on you as a Mega Gnar between the BoRK and her Hiten style active. So long as you don't let him get his stacks on you, it shouldn't be cake but it should be doable.
Jax jumps on top of him, stuns him, and eats him alive.
Any assassin in the game gibs him before he has a chance to leap away. The skilled assassins can catch back up even if he does.
Take some hard CC with you if possible, maybe even Exhaust instead of Ignite if you're THAT worried, and pick someone who can either deny him farm or use him pushing the lane against him. Get an early gank from your jungler to force him from lane, and start setting him behind. If you're the jungler and YOU don't want to deal with him in the mid game, gank willingly, work with your top laner to set it up.
Most of the time people are QQing about Gnar it's because they didn't have a team that could handle him because they either A) weren't mobile enough, B) Didn't have enough CC to stop him, or C) Didn't have enough burst to kill him in mini Gnar form.
Go brush up, chill out, and try again next time you see a Gnar. That, or use my personal strategy for learning how to beat a champion, put a day or two into learning them inside and out so you know what to expect from one.