REMEMBER!
Gnar is not a tank! He's a fighter! The only sources of damage are Trinity Force and Blade of the Ruined King! The remaining items must be defensive!

Basic fighter runes.

Basic fighter masteries.

Try to harass with Q and basics everytime you can. When you need to stop the enemy and you can't turn to Mega Gnar, jump over him with your E and you'll be able to deal more basic attacks to him. Don't throw the boulders in Mega Gnar form just to harass. The cooldown will apply to the boomerang if you don't get the boulder back. While in Mega Gnar form, after each skill, do a basic attack because of the Trinity passive, which will deal tons of damage.

+Not focused in teamfights.
+High mobility.
+High damage output.
+Doesn't die easily.
+Tons of CC

-Vulnerable against CC.

I will update the guide as soon as I get new ideas. Be patient. You already have the necessary information.