I would recommend using Attack Damage marks instead of Armor Pen for early trading and last-hitting. I wouldn't worry too much about who your opponent is in most cases. AD is typically preferable early-game and Armor Pen is typically preferable later in the game.
If people are purchasing attack speed items on Gnar, it is typically to take advantage of his W-proc. You get a lot of attack speed in mini-form without any items, so you don't really need to itemize for it.
can be valuable on Gnar since it provides attack speed for his W, mobility, and he can easily proc Spellblade with his Q. I don't like buying it on him since I find it leaves me pretty squishy (it's quick expensive and doesn't provide much durability), but it isn't a bad option.
Good rush item against melee AD champions (i.e. Riven, Wukong, Rengar) for early durability and damage.
Provides good bulk, armor reduction, CDR, and mobility. Very strong item on Gnar, especially when fighting tanks.
Great item on Gnar. It allows you to easily chase/kite enemies in ranged for and to peel/stick-on enemies in melee form while making you more durable.
Hexdrinker is a desirable item in certain matchups, so you would finish it into Maw of Malmortius at some point later in the game.
Good general defensive items. Gnar is typically built as a tank, so pick and chose your defensive items according to the situation.
My final build used to look something like this, though I haven't place him since about 5.14.
