I just discovered that Gnar is pretty cancer...

With a big stick, preferably.

In seriousness, it really depends on who you are playing.

You spelled Teemo wrong OP.

Must play around his mega form.

Gnar doesn't have many hard-counters due to how flexible he is; he can max his Q or W first--or distribute points fairly even between the two--and crank out quite a bit of mixed damage against most toplane picks. He also has a huge variety of items to choose from, and often builds-to-counter whoever he's laning against--some players will open up with a while others might opt for an early .

Just pay attention to his build as the game progresses and try to capitalize on any holes in it. If he doesn't have a lot of CDR or HP--for example--then he's going to be pitifully-vulnerable to strong all-ins, while a very tanky build will crumble against a lot of sustain.

If you get counterpicked in champion-select, play ultra safe and scrape up as much CS as you can. If the enemy toplaner picks gnar before you've locked in, try to respond to his pick with abother flexible toplaner who isn't as prone to being kited. And always try to keep tabs on what skill he's maxing first--if he's dumping everything into Q or W it should be quite apparent.