Hello, this is my guide to Gnar. Here in my guide I will recommend what I think is to be the best customization and way to play him with item, runes, and mastery suggestions.

Pros: Strong Crowd Control Good Poke Percentage Damage High Mobility as Mini Gnar Strong Burst and Defense as Mega Gnar

Cons: Short Auto Attack Range Squishy as Mini Gnar Slow as Mega Gnar Can't switch like Jayce or Nidalee Slow Early Game

Greater Quintessence of Armor Penetration Chances are your opposing laner will not start with very much armor, so this will help better than more attack damage.
Greater Mark of Armor Penetration These marks together with your Quints will leave you at 19.2 armor penetration
Seals of armor are great as it will reduce your opposing laners damage.
Scaling magic resist are better than flat magic resist glyphs as they quickly become stronger than the flat.

This item synchronizes well with Gnar because of Hyper, and its active allows for unexpected outrading.
With low enough cooldowns to proc it and already strong burst, Trinity Force is an ideal item on Gnar as it allows him to kite with the additional movement speed and extra damage from the passive spellblade proc.
Randuin's Omen gives Gnar some much needed health, armor along with slowing the attack speed of any of his attackers, and the active allows for emergency slow to keep your enemies from getting away or getting to you if you're low.
Most of the teamfights should be revolving around you, so while your enemies group up you can burn them together. :D
Spirit Visage gives great stats; some nice health, magic resist, and health regen, but also allows for you to heal for more and cooldown reduction helps you proc your Trinity Force more.
Against high crowd control teams, this would be the way to go. You don't want to be that guy who accidentally initiates a teamfight while your adc just recalled because you got hooked by a Blitzcrank.
Easily one of the greatest boot choices, especially versus high crowd control teams as tenacity allows you to get free of crowd control to continue your rampage.
Against a team consisting of mostly just auto attacking fools, Ninja Tabi allows for a lot of damage reduction.

Well, I hope my guide helps you to the ladder of success in playing Gnar, but in case it doesn't I invite you to come over and share some Total Biscuit of Rejuvenation. I don't like to share, so that is how confident I am that if you follow this guide and aren't a total nincompoop, you will be successful.