https://www.grimtools.com/db/items/13824
Does this item actually drop? Is it an ultimate only item even though grimtools doesn’t say that?
Only the mythical version is stated as a drop from Gurblix in the monster database, at a 4% drop rate.
I’ve killed Gurblix 12 times on normal and 40 times on elite difficulty now, leveling from 71 to 75 just by grinding this and it still hasn’t dropped. I’ve gotten a few other lvl 75 legendaries from him though during these runs.
Starts dropping at boss level 73 according to grimtools.
But well, drops can be weird at times. I once couldn’t get a green MI item with a certain level even though GT was saying I should and odds were vastly in my favour. Same story as yours basically - grinded from 75 to 76 and didn’t get a green MI with about 20% drop rate.
Looking at grimtools world map as well, Gurblix maxes out at approx. Level 75-76 on Normal/Veteran and Level 90-91 on Elite, so you should still be able to farm the belt on either difficulty.
I can’t speak on the Level 75 version but I have farmed the level 94 belt on Ultimate before, took a few days but it does drop. Just keep at it, you’ll get it eventually 
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Just found this topic - I did 18 runs on Ultimate, with a level 100 character, and it dropped.
Can confirm - I got it recently as a very lucky drop from a “routine” runthrough in Ultimate (level 100) for the Seal of Morgoneth, wasn’t even looking for it.
