To an extent, I agree with you, but using primarily Riven, Fiora, Shyvana, and Irelia top (all of which counter Gnar) I will say that he isn't op.
He's on the strong side of balanced in the right hands. The major killer I've noticed is the boomerang hitbox and speed. Because of the buff it's noticeable post-patch how he can dominate with pokes and outlevel you. Before it was a rare skilled player who was able to poke well and own lane.
The key to countering Gnar isn't necessarily the champion itself (or "herself" in the case of all my top-laners,) it's gauging openings and knowing when to jack the throttle to max. It's more of a skill thing. Having been practicing top lately, I find many of the openings wouldn't seem like openings to safer players.
"Oh, my health is 40%, his is 100%. I can't go in on that." That's what I used to think, then I'd run, then he'd slow with the boomerang and I'd waste a Flash.
Now I'm thinking "My health is at 40%, but my shield should cover autos and his Boomerang is on CD." E, Q, W, (Hop fail on Gnar's part) Q, Q, stunned, down to 30% health, auto +passive buff, ignite if needed. Rinse repeat.
The one level difference you'll get from the kill, if you keep capitializing on those opportunities, is an advantage he cannot overcome without help. He's too vulnerable and with Riven and Fiora, one level difference is distance in miles with a squishy. Sometimes, if you have a jungler with CC, you can even take him in mega form. I went 14/2/14 recently against a Gnar.
It just takes practice. With Fiora and Riven, getting close isn't much trickier. With Fi, use the Q to a minion between the two of you, and Q again to him and hit him with exhaust or all-in with ignite, with Irelia, do the same, but just last-hit a minion with your first Q so it auto-refunds it for the second Q to Gnar.
It just takes aggression and having the courage to go in. He won't outdamage you as mini-Gnar unless he outlevels you. The key is to make sure he doesn't. A good gank should fix that if he does.
He's balanced, he's just very on edge of being overpowered and underpowered. He's on a very delicate balance. Any new top-laners could easily throw his balance way out of whack. He's in a perfect sweet spot right now.