Conclusion and TL;DR at bottom.

I do this guide because I see all guides advice on items I believe are bad for Gnar.
This is happening because I think there is a misconception about how to use him most effectively.
I will explain what I see as the correct way to play him.

You see, Gnar has a bit of a bipolar disorder.
His two forms, are completely different, and as so, he must play completely different.
If you buy a triforce, yomus, or botrk, at the beginning, you will be useless in your both forms, until you get either some real damage, or some resistance.
Let me explain:

Minignar has a core ability, which is pretty much op if you master it. Always catch his boomerang please. It has a good slow, and is always available at max rank if you catch it. The problem is that it does not deal much damage. If Gnar doesn't get some flat AD along the game, his Q will decrease in effectiveness over time. The friggin thing has a 100% AD ratio, plus the base damage, which is 150 at max rank. That is insane. The Triforce rush, and BOTRK, are intended to explode Gnars potential with his HYPER ability though. Trust me, it is not worth it. You are rarely going to be at autoattack range safely. If you are, Hyper is OP enough to do huge harass without a Sheen passive or bonus attack speed, specially since his Q procs Hyper too. So, as minignar you want to be as away as possible most of the time, poking with your Boomerang. In order for the Boomerang to deal a damage that is worth it, I buy a BloodThirster first. It helps huge with farming too. You will surely see Gnar feels different when your enemies start to actually fear your boomerang. Specially when you throw it nonstop. I guess you could try an IE instead, or a Maw. But I like the sustain BT gives with the lifesteal. BOTRK gives sustain too, but you don't get that nice AD. BT gives you an instant +80 damage on your Q.
Hyper and Hop, you want them for getting away, or closing distance on weak opponents to finish them off. Poke them with your Q, until they are low enough to jump on em and chase.

Lets talk Megagnar now. The true menace in teamfights.
When your Gnar gets angry, you want to act completely different to your minignar. You may want to keep your Q if you cant pick it up. Mastering the hop combo into MegaGnar is crucial to engage on enemies. If you have no rage, you may want to stall teamfights until you build enough. It hurts me to say this, but its true: MiniGnar is useless in teamfights. (Unless poking with his Q safely). I realize the Triforce, Botrk, etc. builds try to fix this somehow, but they just cant do the trick. You win more by building resistances after a first big AD purchase like BT. In mega form, your Q gets a 120% AD ratio. That is 96 extra dmg from BT. His Ultimate also has AD ratios.
This is a bipolar build. So you want damage, but you also want lots of armor, health and MR on MegaGnar, to become the absolute monster he is.

Gnar's bipolarity shines on teamfights too. If you are ever fighting against a Gnar, engage when his rage is down, and kill him quick. If you don't, and don't take down their carries fast enough, be prepared to deal soon with a monster that will likely turn the fight around. He is great for baits in this way. However, MegaGnar just falls off mid-late game if he doesn't have defensive items. Getting 2 or 3 damage items at the beginning kill MegaGnar's value as tank and CC. But if you start with the defensive items, MiniGnar becomes useless at the earlygame. So start with AD for MiniGnar, after that, resistances, and when the tank build is ready MegaGnar is a big big force to deal with.
Banshee's or Visage are great. I favor Thornmail over Randuins, but you could experiment your own flavors on items. I like to just have one armor item (outside maybe boots), to leave space for 2 damage items and the MR items. Maw is perfect. It makes Gnar's Q even more dangerous, and gives that nice MR. Squishy targets will have a bad time if you poke them. Make sure to have some defenses first though, before getting Maw. If you get AD for 2nd and 3rd item, MegaGnar falls off badly.

Id leave Warmogs or Maw for last. Try different things. Like Randuins first than Thornmail, and Thornmail in the end instead of Warmog. Boots to your discretion. Runes and masteries too. I'm not here to tell you what to do.
Just to share with you the way I believe takes the most out of Gnar's kit.
Everyone seems to not understand the strengths of MiniGnar, and this causes a build that ends up hurting his potential. Both in mini and mega.

Conclusion:

The bipolarity brought to Gnar with items like BOTRK, Triforce, Yomus, and even drunk people that buy Last Whisper and Wit's End, is just not enough. Gnar needs to favor AD over Attack Speed. He is kinda useless in miniform, we have to accept that, and let go those AttackSpeed items. They only hurt his true strengths. He can actually be a great poker with his boomerang, but he needs AD for that. And since he needs the resistance items for MegaGnar, we have even less space for Attack Speed items. When I see a Gnar build a Zephyr, Wit's end, Yomus and Last Whisper, I cry. I'm not against breaking the meta, but people actually try to make him work like that, like being serious about it. That's not how it works. I've won games with AD Malzahar, but I know that's not how he works. Gnar doesn't work that good with HYPER and Attack Speed. You can do it, but its not best.
MegaGnar is one of the scariest and strongest tanks in LOL, if not the strongest. This is balanced by MiniGnar's weaknesses. This don't mean you should try and make MiniGnar stronger. You'll make the mighty MegaGnar weaker by doing that. Focus most of your build on his natural strength, his megaform, and time your transformations wisely.

TLDR:
Minignar's HYPER is more useful as utility than damage output, therefore, favor flat damage over attack speed. It scales really good with his Q damage, Minignar's true poking strength. MegaGnar also benefits a lot from it, having even higher AD ratios. After getting some damage, focus on Megagnar's Tank nature and build resistance. Learn to change your playstyle to the will of Gnar's transformations. From a cute Yordle that pokes from afar and is slippery, to a powerful mountain that wrecks everything around him.

I hope this helped. Tell me what you think in the comments if you like.