Against gnar, Gnar can easily escape ganks with his E. Try and bait this ability out before your Jungler arrives in your lane to reduce his chances of escaping.

Do not commit to using your Ignite or any burst spells if Gnar is about to change forms as Mega Gnar gives him extra health that will allow him to survive. 

As he is a ranged champion, it’s important that you do not let him poke you down and harass you with basic attacks. Be prepared to sacrifice some CS for XP if you’re low. Against gnar, Gnar can easily escape from the ganks with his E. Try and bait this ability before your Jungler gets in your way to reduce his chances of escaping. Do not commit to using your Ignite spells or brilliance if Gnar is about to change shapes like Mega Gnar gives him additional health that will allow him to survive. As he is a varied champion, it is important that you do not let him down and harass you with basic attacks. Be prepared to sacrifice some CS for XP if you are weak.