I play Gnar a lot, Gnar is a really bursty character with hard CC in his Mega Gnar and can poke and kite a lot in his Mini Gnar, so I like play him when ever I get the chance, but I go AD. I go AD because of how I don't think it's affecting the play that much when you go bruiser or tank, the only thing that I pretty much sacrifice is E so I build frozen mallet mid game that's when I build up the E. In my opinion I think that going AD Gnar is a stronger choice than bruiser or tank Gnar which you only capitalize on your passive and E.

I like to go 2 quints of AD 1 of attack speed, Full marks of AD and both seals and glyphs of attack speed due to your W I think these runes will give you better trading in the early game depending on how you play and trade with him. For masteries I go the average 21 and 9 with the double edged sword for increased trading.

Ranked play you might get picked on for this build because of them asking you to build tank. If your team only has a tank like Braum you might wanna switch this build up with a Randuins for a Youmuu's ghost blade or something. Personally ranked play might be a strong pick against a Jax or a full melee top like Renekton, Darius, Garen, Voli etc...

Buy a Blade of the Ruined King first because you'll have that more sustain but also that extra attack speed for your W after that you'll get the trinity force with that extra speed with that phage built into it. Frozen mallet for the third for more kiting with your W and Q plus the chasing with the Q and Mega Gnar Q and you can do a little more damage with your E. Zephyr I chose because of how much attack speed It has, I think that it's better that the phantom dancer for you because it has more movement speed and has tenacity for hard CC from Veigar or Zeds slow. Lastly the youmuu's for the extra attack speed, armour penetration, spamming your Q and affecting your W much more with the active.

For the basics if you don't even know Gnar's spells are then here you go this is a guide anyways so...

Rage gene: Over time Gnar builds up fury when in combat by dealing or taking damage once his bar reaches 100 Mini Gnar will have a red glow on the screen that means he's going to turn in to Mega Gnar
Mini Gnar: Mini Gnar has extra movement speed, attack range and attack speed.
Mega Gnar: Mega Gnar has extra health, armour and magic resist, health regeneration and base attack damage

Boomerang throw: Gnar's Q is a boomerang he throws but he it doesn't hit back like Sivir's after it hits a unit it slows the target for 2 seconds if you catch it the cool down for the boomerang will reduce depending on how much you level it.
Boulder toss: Gnar throws a boulder slowing the target(s) and dealing massive damage. Plus Gnar's 120% AD ratio when throwing a boulder.

Hyper: His W is a speed boost and extra damage if you hit a target three times. His W does magic damage and 100% AP ratio and plus the target's max health.
Wallop: Gnar slams the ground stunning the target(s) in an area dealing damage plus 100% bonus AD damage

Hop: Gnar hops to a certain spot, if he lands on a unit he bounces farther. If it's an enemy he deals damage plus 6% of Gnar's max health. When Gnar hops he gets a small attack speed buff for 3 seconds
Crunch: Gnar in mega form he deals damage in an area plus 6% of Gnar's max health. You can hop and crunch when your about to transform in to Mega Gnar when the E icon turns blue.

Gnar!: Gnar's W (Hyper) has increased movement speed.
Gnar!: Gnar tosses all enemy units in to a certain area slowing them and dealing damage plus 30% AD ratio and 75% AP ratio , if knocked into a wall or turrets etc... they will be stunned for a few seconds

I like to get Q first for poking in the early game, get your W afterwards. Usually you'll turn into Mega Gnar at level 2 so when you turn into Mega Gnar you'll have hard CC during level 2. Get your E afterwards because you need an escape. Max your Q first because during Mega Gnar you do bonus 120% AD damage and you can do quite a bit of damage with that boulder. W is next with hyper, hyper's a really good passive just like Vi's W or Vayne's W but gives better kiting than Vayne's W. Your E is for maxing last because your not building bruiser except for a frozen mallet. Ult max right away like all champions. A good combo is when your about to turn into Mega Gnar then you E to the back lines shove the team into a wall toss your boulder then wallop then keep on DPSing them.

I've thought about this and if you want a amazing team wombo combo comp then you'll have Oriana, JarvanIV, Lucian, Morgana. The way the full wombo works is that morgana ult's stunning the whole team oriana goes for the shock wave while all of them are stunned and now more stunned. Jarvan goes in and ults while there stunned then Gnar goes in as Mega Gnar and Ults them all by Jarvans wall ult then Lucian ults while there completely stunned. Stunned for days! I'd say Gnar, Jarvan and Oriana are a good wombo team all together this is good during 5v5 ranked or remade normals.

Pros Cons
Gnar is amazing at kiting Gnar has trouble disengaging during Mega Gnar

Mega Gnar can chase champs easy You have to use your Mega Gnar at a good time for example
about to turn into Mega Gnar TP bot lane

Very hard CC as Mega Gnar mid If you don't catch your boomerang or grab your boulder you
game get a long cool down

Good poke during early game

I think Gnar is a really good champ just because he has the DPS, he has the CC, he has good kiting it's a really good champ I'd say the people complain about his fury bar but you can have control by stop attacking when your rage is at 90 it stops pretty close. I've been able to control it by stop attacking then wait for awhile for your rage to go down you might miss some creeps but later on it's pretty fine after you'll get kills