So how exactly does one win lane against Gnar?

to name a few. Wait for him to go mega, wait for it to run out, and then engage to either run him out of lane or get a free kill.

pick Irelia...dodge boomerang. Q onto low hp minion by gnar. E+AA and when he hops away Q onto him. Laugh as gnar dies or is forced to base. (Note if Gnar's bar is like 80% full do not do this...he will go mega and kick your ass unless you are really ahead.)

or pick Fiora and just skip the low HP minion part

Or pick Darius and pull the little guy in when he wants to cs and chunk the hell out of him

Or Jax

Or Hecarim

or Pantheon...perhaps just any strong melee champ that can force gnar into melee range or poke him at equal range

Until my last game I hadn't lost lane to a Gnar since about a month after he came out, with Garen being my main pick against him. All it takes is knowing when to fight, when not to fight, and how to dodge a skill shot.

Basically anything with a gap-closer and playing smart, to sum up the previous posts.