Gnar is a squishy/tanky toplane fighter with good poke ailitites and the potential to carry games with ease. He is a prehistoric Yordle once frozen (literally) in time. Now he has escaped, he is normally happy and cheerful but sometimes he goes ragy supamonster, and destroys stuff like champions and turrets.

when playing Gnar it is essential to manage your rage; it goes up very quickly and mega Gnar can save you from potentially lethal blows, but is unable to escape, so you would want mini Gnar when running and mega Gnar when massive damage is incoming, if you let yourself turn into mega Gnar whenever this will often not work.

Mega Gnar is tanky, slow, does high damage and has lower attack speed, mini gnar has the complete opposite of each o fthose except damage; he can still do alot but mega Gnar gives extra damage.

Gnar is a good pushing champion (mega Gnar does so much damage to turrets it's unbelievable)
and works well in AoE wombo combos.

With Gnar it is important to have Attack Speed, Attack Damage, Armour, some lifesteal and movement speed.

Mini Gnar

In the early game you focus on two things: farm and poking.
To poke, simply use your boomerang; don't engage properly as you're squishy. This will often yield enough damage to eventually push them out of lane if you hit every time, and catch every time. Sometimes you can throw in two basic attacks aswell to trigger hyper (w). You want to try and stay in lane as long as possible, so buy health potions and also buy damage to keep up the poke. Often you will rely on ganks to get kills but this is fine; just get one or two ganks from the jungler. If you let them push a bit into the middle of the lane this will work as you have a slow.

Mega Gnar

If you're about to turn into mega Gnar try to manage your rage to stay at about 90. let them push into the lane a bit and surprise them with a Q,W,Q and if you can R. to close the gap use E as the first move to transform into mega Gnar, then follow up trying to deal as much damage with abilities as possible. When they die if you still have decent health you can push, if you have less than 500 health, you had better go, as you will get ganked; this often happens to me.

In the mid game from lvl 8 onwards you start to get tankier, do higher damage and become more dangerous. Around now you want to try to get the Blade of the Ruined King, and ninja tabi WITHOUT an enchantment (it costs too much and is not necessary).

You want to push hard around now, try to get as many kills as possible in order to get fed (you can now attack with harder), and push more easliy. If you can push hard, do, and make use of times when they're not around. Remeber that if you get fed you can offer help to other lanes at this point.

I think that level 13/14 time is Gnar's prime point, when he can really do sustained damage without too much oppsotion. He starts to be able to attack with unrestrained agression about now, and this is when he is really scary.

Back on your lane, poking and farming is important, but farming now to push rather than level up, as hopefully kills aren't too difficult to get. Remember the Mini/Mega tactics from the early game.

In the late game you should have many late game items such as Randuins, BoRK and TriForce, you should also now be able to attack hard, push very hard and win teamfights. You may be able to solo Dragon, and should be tanky enough to not have to go back to base that often. When killing ,mini Gnar should be enough, it should be easy to whittle down with frequent basic attacks and boomerangs.

To kill, engage with a booomerang, then hop too them, follow up with two basics, a boomerang and finally whatever else is needed to finish them. This works if they're not overly tanky.
If they are tanky, you want to use mega Gnar to stun them and do more damage.