You want the wave near your turret until you have first item. Gnar has a lot of movement speed with his double jump and hyper procs. Try not to push the wave when you are farming with q.

3Q>E max Stand behind minions but not right behind them so Gnar’s q cannot hit you. Avoid getting hit by Gnar’s hyper. Ult when he stuns in Mega so he cannon combo it into his ult. Be careful when he is about to turn into mega form from mini. Dshield and second wind helps with sustain against his q. If you really struggle against him Fleet Footwork might be beneficial to help you survive.

Would not recommend taking early trades against Gnar. His stats as a bruiser are simply higher than yours in the early game unless Gnar misplays you will not achieve a significant advantage in lane. Look to trade when Gnar is in his Mini form but be sure Gnar cannot extend the trade and run you down the lane with his hyper movement speed.