Blade of the Ruined King is a good start from there build tank. They have lot of AP Locket of Iron Solari is good, Banshee Viel too, DeadMan's Plate is a new item that would be nice. If you're team is doing good Sterak's Gage would be awesome since it buffs your normal attacks by a good amount and has a nice passive as well. Hope that helps!
I used to play gnar when the season was fresh and the way i built him then was just pure tank with maybe a Hexdrinker.
Nowadays i see most people go Black Cleaver into Frozen Mallet into Tank items. You can go blade if you are super fed and are looking to end the game quick. Randuins/Thornmail/Spirt Visage (This buffs your large forms regen passive) are core items as well. Boots are either ninja tabi or mercs depending on their team. Heavy AD vs Heavy CC.
Your main damage in lane comes from your W passive. Procing it with your Q gives an amazing burst. With gnar not having any resource it's really easy to spam this. Baiting people at high rage is also a thing. Remember than in large form with your ult you can stun an entire group for 4-5 seconds while still putting out damage with your Q and E. Controlling it at first is hard but the more you play the more you can get rage form when you need it.