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.
Gnar needs his Ultimate R to be available and the only way he can get it is through changing into Mega Gnar. Try to team fight when he has very little Rage stacked, so he is unable to change forms.
When Gnar is in Mega Form, and you’re about to team fight, be prepared to disengage and call off a fight until he is in Mini Form. Gnar is very weak in team fights when he’s in Mini Form.
Avoid fighting in close proximity and in areas that are tight like near walls, around objectives or in the jungle. Fighting in these areas helps Mega Gnar get a good Ultimate R or a multi-person W.
As soon as Gnar hits level 6, try to avoid standing near walls when he is in Mega Form as he can throw you against them with his Ultimate R and set up his Jungler.
Gnar is really good in extended trades if he can use his range advantage as his W can proc in a skirmish. Try to trade around his Q so he cannot easily proc his W.
Gnar is much stronger once he has his first item, which is usually Black Cleaver. This item will make him much better in a 1v1, and you’ll have to be slightly more cautious if you’re a tank as Black Cleaver offers armor shred and more mobility.