Good build and everything, but I do think some attack speed is good on Gnar - especially on this build. Gnar's Rage works so that when he attacks a champion he gains a certain amount of rage, so you'd obviously want to have some attack speed. Either boots, runes or whatever. Something that could help you get to your mega form faster!
This is just my opinion and thoughts though.

This is a Gem
I ran into this guide and found it very helpful to look at Gnar in a different perspective.

One of the reasons Gnar builds botrk is because it has a very smooth transition in building all the components and the slow is nice for 1v1 in kiting. No kidding, I've kited a Leona from 100 to 0 at lvl 7 with merely a bilgewater and no boots, taking no damage.

It may be an extreme example but you get the point.

Imo it's still may be a better idea to get Frozen Mallet over Mogs, if you get it at all. I still insist on Triforce being a core build on Gnar though, because inf edge with no as doesn't really make the cut.

I think you should come up with some builds that vs ad heavy/ap heavy
like vs ad heavy go bt/thron/tabi/tri/mallet/bc(maw) maybe
and vs ap heavy go bt/maw/merc(tabi)/tri/mog/visage
The reason for such heavy differences (aka not building any mr/armor for the later) is because I feel building for 4 of the enemies is just better, not to mention that Gnar will probably kill that guy 1v1 anyways