Well I've been playing Gnar on ranked and normals so I made this guide according to builds that I tried out.As Mini you have to abuse enemies with Hyper and Boomerang Throw and when you go to Mega you just Crunch and Wallop some enemies.

Only alternative would be Teleport but its less useful than Ignite,you wont have that much AD early on so you ll need a way to make sure enemies will die.
Flash is a must have on this kind of champions because of the Mega form that reduces your jumps range significantly.

Just use your ADC masteries cause you will need damage more than sustain in early game.

You'll have to time your Boomerang and Hyper carefully. Also ,if you see an opportunity, harass enemies with Hyper.

So yea this build looks pretty damn weird...Well the thing about it is that its using all of Gnar's strengths and it neutralizes some of his weaknesses.

Pros:
-Nice poke
-Good escape
-Big arsenal of CC
-Ultimate that can change teamfights and ease your escape.
Cons:
-Transformation can't be controlled
-Big casting time in Mega form
-Mega form makes you vulnerable to ranged attackers
-Really squishy and prone to CC when in Mini form.

Gnar is the kind of champion that works pretty well with AoE casters.You can use his ultimate to throw enemies in an ally's Crowstorm or Shockwave. He is also good at peeling when in Mega form (Wallop) and when in Mini form he can act as a secondary ADC due to his W and big base attack speed.