Gnar can easily escape ganks when he has his available, try to bait out this ability then make an attempt to catch him with your Jungler.
His boomerang flies back in the direction he is going and reduce it's CD while caught. Keep this in mind whenever you want to harass him.
His Mega form gives him extra health. Don't use Ignite and let him die when his is about to transform because you'll waste it and he will survive.
Gnar will be looking to use his in teamfight. It slows or stuns (depends on if you hit a wall). Avoid being stunned by not fighting in enclosed areas like inside the Jungle.
Mini Gnar is one of the best kite machines against melee champions but while in this form, he is very vulnerable and squishy. Do not try to chase him without any mobility or ranged as he will always outrun you,
Gnar takes time and has to be proactive to change forms. Engage a team fight when he is unchanged to reduce his effectiveness.
The Black Cleaver and Frozen Mallet power spike is devastating and allows him to kite and poke way easier. Do not over extend as he can easily shut you down.
As soon as Gnar hits level 6 try to avoid walls when he is in Mega form as he can throw you against them and set up his Jungler.
Mini Gnar is squishy. If you can catch him in this form, you will have a higher chance of killing him- but take note, he is more mobile.