Gnar is a bruiser top lane who can do a lot of damage and at the same time outputting a lot of damage and has amazing CC that at least last for 3 full seconds on a successful full burst. He is rather picked or banned a lot in high tier solo queues and in the LCS.

When Gnar was released people thought that Gnar will never be played competitively in solo queue due to the fact that Riot was releasing hard to learn easy to master champions. That made Gnar difficult to learn with his passive bar and how to use it effectively and almost felt like switching back and forth to a different champion every time. Over the course of time, he's been able to understand more and be played competitively and also able to time his passive. He's been getting some nerfs in some patches sadly and if nerfed even more, Gnar might be taken out of competitive play as did Lucian.

For runes I go for armor and magic resist with health, and add a little attack speed to add with his W. If you want you can go scaling runes, personally I go for the normals because I think that the runes will be less useful later on. The runes will only help in the laning phase, but if you want scaling you can. For masteries take 9 21 and 0 very average nothing complex but if you want a little more damage early game you can go 21 9 and 0 but it's not suggested if you want to be tanky and do less damage.

Quints
Armour
Magic Resist
Attack Speed

Marks
Health

Seals
Armour

Glyphs
Magic Resist

Flash: Always the spell you want to carry in battle I don't suggest going without it like tele and ignite even though you have E to jump away I suggest carrying this since if you get ganked while your E is down you might have a bad time.

Teleport: Getting this is pretty important if you know how to use teleport effectively or your in a pre-made like 5v5 or even have really good map awareness I suggest getting this so you can work on using teleport effectively and help out the other team. This is also useful so you can TP down dragon if there's gonna be a team fight there.

Ignite: I suggest this only if you suck at using TP effectively, don't have good map awareness or you're just a selfish prick Kappa jk. This is basically the opposite of what I said on teleport

I want to put this where mini-gnar and mega-gnar have their own pros and cons.

Mini-Gnar
Pros

- A kit for kiting and poking really well
- When adding frozen mallet his W becomes really strong
- Boomerang is a really good poke tool
- Able to escape from ganks pretty easy

Cons

- Very bad when Mega-Gnar wears off in team fights
- Isn't able to lock the enemy team down when in Mini-Gnar form
- Feels useless when in team fights in Mini-Gnar form

Mega-Gnar
Pros

- Amazing CC
- Really good 1 time burst damage
- Pretty good tank late game
- Really good gank set up
- He's able to chase with his boulder
- Team fighting is really good

Cons

- Very slow, vulnerable to ganks
- Mega-Gnar last for only 15 seconds, needs to use it wisely
- Mega-Gnar can be difficult to time

I suggest playing him in ranked, he's got really good potential against other top laners and able to zone them out. He's been played competitively in most ranks. He has really good team fighting, and laning phase I suggest picking or buying him to learn. He might be difficult to learn but you'll probably get the hang of him in about few hours maybe

Gnar is now my favourite champion to play now in solo queues, I carry a lot in my games even though I'm pretty sure it's just because I was demoted due to the reset but I really suggest playing Gnar he is a fun champion to play, he might get nerved even more as time goes by so play him while you got the chance while he's still viable :)