It definitely works being an adc as gnar. How, may you ask, does it work. Well then let me teel you
First of all Gnar can be used in all lanes not just top Mid is when you want to be a deadly b#### and bot is when you can be a nasty tricky and slippery b######. Being an adc is deadly as gnar, especially when you are Mega Gnar because I played Gnar as mid lane and adc for almost the whole time and I almost won every game. Gnar has amazing tricks with the boomerang like curving it, after when you throw it move over to a point and you will see that the boomerang follows you so then when an enemy is behind the tower then use the curve and move to the side infront of your enemy (note: throw the boomerang to the opposite side) then move out of the way and let the boomerang go and hit the enemy to get the kill. The other tricks are on youtube (search Gnar combos and tricks).
So it is 100% possible that you can play Gnar at Top, Mid, Bot, ADC and as a Jungler.
Try it and see what happens.