The purpose of this build is to exploit Garen's W passive, at full build you will have about 300 armor and mr with ONLY 2 ITEMS. It's quite stupid really. I haven't tested this yet, so i would be happy if you'd like to give some feedback. Things are subject to change at any point if any item seems more useful than others.

These 2 items is what will give the resistances. Let me explain, Garen's W passive gives him 0,2 armor and mr for every unit killed, maxing out at 30. After reaching the max, it will grant 10% bonus armor and mr. Gargoyle gives 60 armor and mr. One thing that people seem to forget about hull breaker, is that it gives 60 armor and mr at lvl 18 when you're alone. Combine these 2 items with Garen's W, you get 150 armor and mr + 10% armor and mr (15% with conditioning). With Garen's base stats, you basically end up with a **** ton of resistances. If you don't like the idea of being useless in teamfights, you can ditch hullbreaker and buy an HP item, like Warmogs or Spirit Visage. You could even forget about Gargoyle and buy Thornmail and Force of nature. Maybe you wont even need magic resistance, it all depends on what YOU want to do, and what the situation calls for.

Since Hullbreaker and Gargoyle give so much resistances by themselves, we have to dedicate the rest of the build on DMG and HP. Kraken Slayer gives the most damage, 65% attack speed is nothing to laugh at, specially since Garen's E scales with attack speed. It gives the most damage, but shieldbow is also really good too, they're are both just as good at their job. Bloodthirster is there to give damage while also compensating for the lack of HP, since you have low HP but have a ton of resistances, you will heal more and lose less health.

Infinity Edge is really good if you want to double down on the concept of "**** hp, more damage = more healing". Warmogs can be good if you want to do the opposite. Spirit Visage is good if you want HP and healing.

Divine Sunderer might be good if you want a more bruiser like build, Ravenous Hydra would probably be better than bloodthirster in that case. Since DS's healing doesn't change with the opponents armor, you will also heal a lot with it. Spirit visage would probably be the best 5th item with divine. Trinity and Stridebreaker can also be good for a bruiser build.

One of the biggest problems i have with this build is the pathing, you first build damage until you have W stacked, then you do a 180° turn and go for resistances, and then you go back to building damage at the end. It's gonna be a weird early game.
Another problem is that, since Hullbreaker's passive only activates while you're alone, you might not be as good in teamfights, so maybe stick to splitpushing with this build. Also since you lack HP, high armor pen champs will be a problem.