It could be worth experimenting with Overheal and Last Stand since Gnar has a fairly low Cool Down for all his abilities early. Even catching your boomerang only takes it down to about 10 seconds early in a match. Also I've found that Demolish can be a lot more useful than Overgrowth since you don't usually get a lot of time to safely attack a turret, but maybe play Gnar adc helps that problem.
You could also try making the secondary runes Domination with Taste of Blood and Ravenous hunter to capitalize on bloodline a bit more.
BTW With the abilities, you'd have to take Q at level 8.