I think I remember Quas go
as the first 3 items he built as he was up against a magic damage opponent such as Rumble. Finishing Hexdrinker and Phage into Maw and BC. He was still pretty tanky thanks to the free stats he gains as Mega Gnar coupled with the health and resists from the items he bought, but he was more of a threat when splitpushing and dueling. I really like this build honestly over Ghostblade or Bork as they all synergize with his mega form as well. GB and Bork are great for mini Gnar, but your major teamfighting power comes from Mega Gnar.
After the sitautional 3 semi-defensive, offense items, I would go for a Randuin's, Banshee's Veil, SV, GA, Thornmail, or Warmog's as the 2 items you will build afterward. I don't like Sunfire on Gnar as you shouldn't really be in range when in mini form and the stats you gain are just worse than Randuin's. Consider subbing out Bork in your build for Frozen Mallet and Sunfire out for Thornmail or GA as another Armor option. You should end up with similar damage and more tankiness for about the same price. Gnar has high AD scaling and has an ability with health scaling so I really like Mallet and BC as you can double dip into the stats they provide. Hexdrinker should be situational in if you face a magic damage heavy opponent in lane, it can help you fight them more effectively, but otherwise I would just skip the item in favor of more HP and Armor, later getting an MR item for the rest of the enemy team.
Here are some basic build orders (follow them cautiously as they are not one size fits all):
Vs Magic Damage Lane:
Vs AD Lane:

You can swap out any of the items you feel are not right for the situation, for example if you feel like you need more Tankiness, skipping Frozen Mallet or BC can be done for a more full tank build. I like going at least semi offensive on Gnar though as I feel like you are more of a threat when splitpushing and have more lane pressure early. If you do fall behind, going for a full tank build would be best as you can be more useful for your team, but if you are snowballing, going for more offense can extend your lead though I think you should still be getting some tank stats for the later parts of the game.