How do you counter a good gnar

When he's in mega form and about to change back engage on him and trade then, you can also trade with him when he's mini gnar but he can kite you and E away Counters:

try to take a champ that has more global pressure or someone in a different role so your teams win conditions dont overlap with your enemies.

even gnars worst matchups and counters are not strictly that bad, the worst a proper gnar will do is go even and then he still has better teamfight. all you can do is try to impact the game in another way, dont try to beat gnar, its a waste of time just like you dont chase singed.