I am a Diamond II Gnar main ( on KR ).
I climbed my way from many divisions with top lane.
Gnar is a cute little bby and a powerful wrecking ball, I think he is pretty balanced at the moment.
This guide isn't meant to be that lengthy, it's just to give you a general understanding of Gnar.
I'll continue to add to this guide as needed.

Pros:

Lots of Harass in lane
Pretty tanky in the late/mid game
Doesn't fall off too hard
SHUGI SHUGI SHUGI
DEMAGLIAAAA

Cons:

Pretty weak in the early game.
If you get CC'd then RIP
You need to try and manage your rage
If you go mega Gnar at a bad time well....RIP
you need to catch boomerangs to make holla holla get doller?

Ignite used to be the meta for Gnar but now he is really good with teleport :>
It provides a lot of map presence for him

Jump in with full rage and W to stun then Ult the enemies into a wall/ a bad position.
You need to make sure your team follows up though.
Try not to get CC'd while trying to engage

Try not to just blindly (you shouldn't do this on any champ tbh)
Just last hit carefully, and use Q to farm safely
Just remember to also try to harass with Q