I'm assuming you mean Gnar has a 100% win rate in every game you've played, with or against him.
Anyways:
The biggest thing is knowing which form of Gnar you're strong agains t and which you're weak against. There's almost always one. In general, little gnar is strong against people he can kite and poke, but weak against champions with hard engage that can get on him. Mega Gnar is the opposite, he's strong against people that have to be within melee range because he has good base stats and lots of CC. He's weak against people that can poke at him and kite him.
When it comes to mid/late game, pay attention to his rage. Don't engage a fight when little gnar is about to go mega - that's his deadliest moment (because he gets the mobility of mini gnar to get on top of your back line before turning mega to tank the damage and CC them). In contrast, the best time to engage is when mega gnar is mostly done - he's immobile and thus unable to get on to your back line, and by the time he gets there will turn mini and be an easy kill. In general, you want to fight mini gnar, NOT mega gnar. You want to poke mega Gnar, NOT mini gnar.
Avoid clumping up as both his W and R are AOE, and with e+flash he will get on top of you if you clump up. Making up miss his Q in either form puts it on a much higher cooldown which will greatly reduce his poke and chase/kite.