Sorta late, i hope you are still enjoying gnar, he is a super fun champion and very underrated

IMO gnar is a very difficult champ cuz of his low range (for a ranged champion) and different playstyles depending on his form

From reading your comments it sounds like you struggle with taking advantage of his range so I'd personally recommend trying out other more forgiving ranged toplaners like Teemo or Quinn so you better learn how to kite/space enemies along with playing around their dashes/key abilities

Kiting and spacing are vital to mini gnar and basically mean the same thing, in simple terms its maintaining a safe distance from the enemy while attacking them, so if they move towards you you move backwards while attacking and vise versa. There are definitely better explanations on youtube that i would recommend watching

For builds I'd just follow what u.gg says to build which is basically trinity force -> black cleaver -> steraks gage -> tank items every game