Starting

Usually you'll start with Doran's Blade because of the extra AD, HP and sustain it gives.
However, if you face a skillshot reliant champ (I.E. Nidalee, Ezreal etc.) you can choose Boots over it.
If you are against a heavy AD champ like Zed or Yasuo and you don't feel like you can manage them on early, start Cloth Armor and get Ninja Tabi as soon as posible.

First Back

On your first back, depending on how you're doing in lane and how much gold do you have, you'll have some options:

First option:

If you have enough gold and are doing fine in lane you definitely want to rush Guinsoo's Rageblade, so get a Pickaxe. If you have enough gold get a Blasting Wand too, but prioritize the pickaxe, because if tides turn you can always build it into a Frozen Mallet before getting Guinsoo's.

Second option:

If you are loosing lane but not being destroyed, you should forget about the Guinsoo's Rageblade for a bit. Depending on your money, go for:
- Jaurim's Fist
- Ruby Crystal plus a Cloth Armor or Null-Magic Mantle (Depending if your opposing laner is is AD or AP)
This is to build a Frozen Mallet later (Don't upgrade yet), and Ninja Tabi or Mercury's Treads.

Third Option:

If the enemy is definitely destroying you, you should aim to get some defensive items so get a Chain Vest or Spectre's Cowl and build them into an item which fulfills your needs.
- Armor options are Randuin's Omen vs Yasuo or crit reliant champs, Dead Man's Plate for some extra mobility and a bit damage, Sunfire Aegis for better trades while in Mega (Thought you'll usually prefer to trade while in mini so Randuin and Plate are better options).
- Magic Resist options are Spirit Visage to get sustain or Banshee's Veil to deny poke and combo-starting abilities ( LeBlanc Akali Katarina)


No matter what option you choose on your first back, some potions and a Vision Ward will not harm you.

Core

This item is core. It gives AD for your AA, for poking with Boomerang Throw and for every ability in Mega form; AP to deal more damage with Hyper and GNAR!, and attackspeed to get even more Hyper procs, alongside with a passive that at 8 stacks deals AOE damage with your AA.

While Guinsoo's Rageblade gives Gnar tons of damage at a low cost, Frozen Mallet on it's side gives him more damage, some more survival with the extra HP, and a very useful passive to chase enemies or even escape if used correctly in combination with Hyper