As a toplane main that frequently plays gnar I have lots of tips for you if you're just starting out.
First of all, gnar is NOT a bully. Gnar is simply ranged, which most of the times leads into you poking them into their death. That however, does not make him a bully because unlike bullies, gnar is fairly weak early game.
Why, you ask? His base stats are lower of a ranged adcs one, has about 400 range at lvl1 which scales back to 550 in the late game and is right next to sona for the lowest hp pool of the game. He needs the ranged poke to be able to survive his lane. His mega form is very predictable and gives tons of warnings to your enemies of when to engage and when to NOT engage. While in mini, anything can easily kill you, while in mega, you can kill pretty much anyone, HOWEVER, this doesn't mean you should go rambo on your opponent as you will stick to mini most of your time and you will have to learn how to control both forms and use them to your advantage which comes to my second point.
IN LANE : Mini gnar is used to whittle down your opponent, mega gnar is used to finish him off. NEVER (with some exceptions) should you use mega gnar to whittle down your opponent and mini gnar to finish them off. Why? Between lvls 1 to 9, your hp pool doesn't surpass 1000 at all, even with a ruby crystal and dorans. If you use mega gnar to engage on a full hp camille for example and you get to knock 50% of her hp, you might think thats good, but in reality it isn't.
She will easily re-engage on you, killing you fairly easily because without mega gnar you are pretty much defenseless. Sure you can try to fight her back but you will have no means of actually stopping her from attacking you or even dodge her damage. Most likely you also used E to engage on her so no escape tool for you too. You just lost a fight because you used your forms wrong and got caught.
Understanding gnar's weaknesses in lane and your opponents' strength is the main key to making gnar work. Gnar is excellent at taking tanks head on at all stages in the game but that doesn't mean tanks can't kill him, it just means gnar has the tools to win the lane.
Heres a common match up :
You've probably seen pro players taking gnar against maokai and doing fairly well, then you go soloQ and see 0/5 gnars against maokai (as I have).
Why is this? Because gnar is weak early game and the pro players know it. You don't try to fight a maokai even-handed, you poke him down from afar, constantly shove his lane so that he has to choose between fighting you or farming minions, manage your fury so that once you're about to transform you can deal a ton of damage while in mini and if he engages on you, stun him and retreat etc etc. Maokai can and WILL kill you if you try to fight him with no fury and no minion advantage against him, not to mention that he can easily make your life miserable if your enemy jungler decides to camp top, so never freeze lane against this champ and most importantly, VISION.
.
After finishing
and maybe toss in a
, you should pretty much auto win any trades with maokai at any stage of the game aslong as you are not 0/5.
Gnar's rush items are always
and
. The rest of the build is usually up to you, being able to adapt very consistently throughout the games.
If you're losing and your team needs another tank, go
with any pair of boots of your choosing depending on the game (Tabis vs heavy ad, Mercury vs heavy ap).
If you're winning hard, you can switch
for a
and still be a teamfight monster.
If you're against, say,
or
, you can instead opt for a split push build, going
and
. This build will allow you to decimate your opponents completely and push the lane constantly. These snowball-dependent characters normally cannot do anything to you and you can 1v2 the jungler aswell if you kno he's coming and you manage your fury.
One last thing, after you transform back to mini from mega, you will spend approx. 30 seconds without being able to gain fury. BE CAREFULL! An intelligent opponent may and WILL take advantage of that. Always play extra safe when that happens, only trade or overextend if you're 100% positive you can kill your opponent or he cannot kill you.
There's tons more to say about this champion but it really depends on your experience with him now, go play him and master him, know his limits and try to get used to his rage.
Always remember, gnar is not a bully, you will die if people jump on your face so play smart, ward your bushes and have fun!
Edit : Sorry posted this twice, had to remove lower one ._.