Managing your Rage is very important. Try to time out your transformations so you can get maximum benefit from both forms.
Position yourself next to walls to bait your enemies into getting stunned by your ultimate.
Know your strengths! Mini Gnar is fast, squishy, and has high sustained damage. Mega Gnar is slow, tough, and has high burst.
Gnar can't gain Rage for 15 seconds after he turns from big to little. Use this chance to engage on his team.
Gnar's animations and resource bar change as he gets closer to transforming.
Gnar's ultimate stuns if he hits you into a wall. Be careful when fighting him near one.