Hello, im waayWest, an EUNE gnar main in low elo silver/gold that plays gnar in the jungle as a fun off-meta pick. Gnar jungle is currently a D-tier jungler which means it can be a troll pick if you randomly choose it during ranked matches. If you want to try this build out then please do so in normal games :)
Gnar jg is weaker and harder to play than laning, knowing your matchups and where they path is very important since you normally want to avoid the enemy jungler early. I normally play gnar as a bruiser or off-tank/tank since you get to do what you do best during teamfights, initiating teamfights with huge CC chains. The guide will therefor go out of a perspective as if you were building either bruiser or tank since the AP or ADC builds are very limiting and wont be used very often.

Normally I start red buff -> gromps -> transform -> raptors -> transform -> wolves -> blue -> transform -> river scuttle.
Its best to crosspath into most matchuos as gnar though as you normally dont want to face your enemy jungler early, if they start blue then I recomend you start your blue as well. In certain mathups you want to contest the enemy jungler though, if the enemy team has an ivern you normally want to either start red directly into blue or simply start blue and wait for him to arrive.
Starting from the blue side works as following -> blue -> gromp -> transform -> wolves. after youve cleared blue you can look for potential ganks or a full clear since the red side has bad synergy with transforming in time for river scuttle. Dont force invades randomly, your E is on a very high cooldown and you dont want to get collapsed on by their team. If you have to choose between a good gank or drake i'd suggest go for the gank since early drakes does very little for the team, only the soul is important.

Ganking with gnar can be tricky since you normally will need an understanding of how your passive works, finding out what champions you can gank at certain points is of course something every champion have to learn but since gnar cant transform by will it might become tricky to know if you can gank or not depending on your rage bar.
I'll give some general tips on how to improve ganking since its near impossible to list all possible gank situations for gnar.

Focus on farming before you hit lv 6! you are very weak in your mini form and you dont have enough cc chaining as mega when ganking, therefor it's usually better to grind into lv 6 where you get that extra kite potential with your W passive and ofc you'r ult. Ofc you should gank lanes where your laners are healthy and the enemy is pushed up, just don't forget to track their jungler.

Avoid high burst lanes or cc chaining botlanes if your team can't/won't follow up on you, generally only initiate if you can take the hit and ult the enemy into your team or wall. The last thing you want to happen is that you go in solo dying in mere seconds, simply back off if your teammates don't accept the gank.

If you get invaded by the enemy jungler and a laner and you don't get backup from your team try and crosspath into their jungle instead, this does depend on you tracking their jungler though and normaly if your teammates have lane prio they usually come to help.
Just dont stand around watching them taking your camps, you will fall behind, find another route.

Your mini form is better agaisnt tanks/bruisers as you can normally kite and get that W max percent damage, squishier targets are easilly killed by your mega form with chain CC. Mega is always the form you want to have against bursty champions in the midlane.

If you build tank then you need to usually initate, if your team doe's not follow up in time just walk away. theres nothing you can do if you use all your abilites as mega or if you get singled out as mini.

You generally want to keep your rage around 75% when contesting an objective or when there is a teamfight about to go off, since you dont want to take lane farm as a jungler it makes it easier to manage the rage, just remember that if you get hit then your rage builds up as well, so dont get poked into transforming. Good teamcomps will bait your mega early, leaving you rather useless later on as mini. If you are building AP or ADC you should play back, letting your team handle the tanking, potentially you can jump in as AP with Zhonias freezing after CC chaining to survive. Before each fight you need to evaluate the situation, what targets to focus, maybe protect the backlane from assassins? Maybe it's a fight predicted to be lost or too drawn out? go steal farm or do other objectives? just remember, if things go south and you cant transform you should just keep taking other objectives since you will waste your time otherwise. Its all up to the specific situation though, dont leave your team without a frontline!

Gnar jungle is not a strong pick, but it can certainly win you games if you know the champion and how to manage the jungle. You might not be the best 1v1 champion, but the ammount of power you hold during teamfights is immense, even if you have lost the jungle! Sometimes it's best to sacrifice yourself so that your teammates can become fed, and sometimes its best to steal kills from teammates who are behind with no teamfighting power.
Climbing with this may be difficult, but as long as its fun and you dont int games, its worth playing.