You're right, I just tested it. I will fix it for the Spin2Win Build since Titan Build favors AS anyways.

I'd say it's generally less favorable to get Iceborn Gauntlet over Trinity Force. Even if it's much cheaper.

Why? Well, you lose AD/AS/HP/5% MS and your spells will deal:
Q = ~168 less damage
E = ~10 less damage per spin, Garen spins 10 times at lvl 16 so you lose ~100 damage if all spins hit.
Remember this aswell, the attack speed you lose from Trinity will affect how much damage you can dish out with Titanic Hydra aswell. So you actually lose a lot more damage than you think.

What would you gain from Iceborn though is 65 Armor and +250 extra useless mana. The value of the armor can be calculated like this (based on off the build I listed, at lvl 18)
With Trinity Force you'll have 210 armor which is ~67.74% physical damage reduction.
With Iceborn Gauntlet you'll have 275 armor which is ~73.00% physical damage reduction.

Let's say a Caitlyn with 380 AD auto attacks you (without penetration):
With Trinity Force you'll take ~123 damage (~276 if crit).
With Iceborn Gauntlet you'll take ~103 damage (~231 if crit).

If Caitlyn has Lord Dominik's Regards your armor values will look like this:
With Trinity Force you'll have 210 armor - 35% armor pen = 136 armor which is ~57% physical damage reduction, Caitlyn autos will now deal: ~163 damage (~368 if crit)
With Iceborn Gauntlet you'll have 275 armor - 35% armor pen = 178,75 armor which is ~64% physical damage reduction, Caitlyn autos will now deal: ~137 damage (~308 if crit).

TL;DR. If you want tankier build you're better off building the spin2win build, the damage you lose from the Trinity is too high to justify the damage reduction you get from Iceborn in my opinion + We already get slow from Dead Man's Plate although not as powerful as Iceborns slow.
But feel free to try Iceborn, in some situations it might actually be good!

Maybe in the future, for now I have no plans on adding AP Garen since it isn't effecient enough. Against Ivern I can't imagine AP Garen would even be a hard counter to be honest.

I will do!
Currently working slowly but steady with reworking most of the guide to make it even simpler and cleaner.
It will be done before Season 9 hits :)
Thanks for your feedback!

Glad to hear :D

Thanks :D

That's totally fine :)