The main reason why I build a hybrid of AP and tank on gnar is due to the fact that his W and ult scale like CRAZY with AP. With a few AP items and some health you can not only be tanky and frontline 4k+ damage, but you have a lot of damage in mini as well as with your ults in mega. One of Gnar's biggest weaknesses is how hard he falls off late game compared to every other tank/brusier in the game. Building AP allows you to keep up with everyone else damage wise, and you are still incredibly tanky.

Demonic embrace is a staple if you want to try out this playstyle. It gives Gnar a LOT of health and sustainability, while also giving him even more magic damage. Demonic allows Gnar to burn opponents with his abilities, which consist of Q and E in mini, and every ability in mega. This lets Gnar be tanky while also doing consistent double burn damage with his sunfire aegis and demonic embrace.

Playing Gnar jungle consistently can be very difficult if you don't itemize correctly. Bami's Cinder is a must have if you want to be able to keep up with other junglers clear speeds. Gnar does not perform very well pre level 5, as his abilities are on such long cooldowns, and ganking with mega for cc can be unreliable. It is best to gank when a safe opportunity arises, or you have the health to dive a laner. After purchasing some tank components, Gnar can become quite the threat. Mega Gnar's early to mid game is incredibly strong, and if you are able to snowball and pick up some AP items, late game will not be any harder for you.

In general, you will always go sunfire aegis, as it has the best damage and health out of all of the mythic tank items. This means that you will be rushing Bami's cinder every game. If you are able to snowball early, picking up a nashors tooth after finishing sunfire aegis would be a great option. If the enemy team has a lot of healing, purchasing thornmail would also be a good second option. If the game state is fairly even, or you want to choose a safe option, demonic embrace is the go-to item.