I only tested gnar as solo top so far,

so far I'm really statisfied with Gnar's sustain/dmg output.

Early: I try to freeze the lane near my own tower to farm easily and ONLY last hit enemy minions otherwise you'll keep transforming in mega gnar wich can make you really vulnarable early game.
until level 4 I keep under my own turret and last hitting and be sure to stay under your turret once you change int omega gnar, since you lose some mobility wich can make you and easy prey for the opposite laner.

once I hit level 4 I often ask for the jungler to gank my lane, once the jungler is on his way I start to lure the enemy to me by hitting him and not the minions once you are about to transform ping so your jungler know you will go in, jump onto the target and change into mega gnar, immidiatly use Boulder throw to slow the target often they will flash on this moment so be sure to see where to he'll flash. once you know where he'll be land wallop on him wich gives you a stun on him. if he won't die yet and you are almost out of mega gnar jump onto him to give him another stun, and trasnform into gnar, than boomerang on him and in most cases you sure got the kill by now.

at all times try to keep using this combo.

once you hit level 6 you can use GNAR! and this adds even more stuns / cc into you combo. use the same combo as previously but Gnar him straight in a wall when he wants to escape, and do the rest of your combo.

by the time you are around level 12 you can been snowballed pretty nicely so far.
at level 12-13 the real fun starts if you had enough farm, since Hyper will deal lots of damage now and wallop gives a more effective stun by now.

from level 16-18 you will be able to 3 hit KO guys with 2k-2.5k hp (and over 100armor/MR)
use the gnar combo: jump on his head transform from mega gnar to normal gnar (yup this seems rediculus but it works!) (1 hit) boomerang on the target (2 hit) normal hit (after you get your crit items) (crit, hit 3)

my scores so far:
Solo queue (Teambuilder) versus some golds: 20/6/5, (won)
Custom: (with some gold friends) 15/2/5 (lost)
Solo Queue: 7/3/5 (won)