On most standard Rammus builds, Iceborne Gauntlet has quite a bit of value. Mana is always very nice on a champion that uses it, CDR is welcome on a tank like Rammus to get his abilities off cooldown earlier, armor is good, and the slow really helps since Rammus has relatively low CC/utility for a tank.
Compared to Zhonya's, the AP and active really don't help Rammus do his job of tanking hits from the enemy team. Obviously Zhonya's is fairly decent if you try a gimmicky AP offbuild, but on Rammus' core builds it the Hourglass doesn't have a place.
That said, Iceborne really isn't that ideal of a pickup. He scales better with health due to the massive amount of resistances on his W. Armor is good too (especially with Rammus' armor scalings), but health is overall a better buy because of those innate resistances.