Your goal with this build is to get as many W passive procs off as possible. Do this in any way you possibly can without dying. These procs will provide a significant amount of damage and a burst of speed. Chain this speed and damage to overwhelm and outdamage your opponents. Stay in the backline, and use your E to burst and reposition as needed. Items like Everfrost can provide CC needed to assist your team in fights.

Try to play safe if you're up against anyone tanky or anyone you're not confident in playing against. If they're squishy and miss any CC they have, go in as hard as you can, especially as you turn into Mega Gnar.

Make sure to stay out of range of your enemy laner and keep your E up if possible to negate the occational gank.

Weave auto attacks in between your CC to maximise the amount of damage you're dealing in a fight, especially with items like Guinsoo's and Nashor's in your toolbox. Maintain vision of the map and keep an eye on your jungler to take advantage of objectives. Your CC can help a ton to turn the tides of fights your teammates begin with their laners.

Kiting, kiting, kiting. Your only goal late game is to kite as hard as humanly possible. If you can do this with your relatively low health pool, you'll be putting out quite a bit of damage.

If you decide to split push late game (i.e. you're behind) just keep an eye on their positions and make sure to buy a Runnan's. You'll be melting waves in no time. If a timefight is occuring and you're behind, focus on chaining your CC as much as possible and provide a way for your teammates to supplement your lack of damage.

* Gnar is very squishy with this build. He has an extremely low health pool and relies soley on CC, kiting, and burst.
* This build requires a lot of focus. If you get caught once, you're dead.
* If you don't have a tank on your team or some heavy CC to peel for you, things will get very difficult very quickly.