Okay, I took a look at one of Potemkin's win poses from GGX2 (where he's saluting Gabriel) to get the latter's height (Gabriel). Assuming Potemkin IS 8'0" and the top of Gabriel's head probably goes up to around his mid pectorals (Potemkin's giant biceps are in the way) then Gabe is around 6'0" to 6'1". My way of finding this was kind of shit so I'm not 100 percent confident in this.
Here’s a better comparison:
When Gabriel was in a Conference Room with Ky, Sol, and Zato in Xrd Sign, he was about eye-level to Ky who was the shortest man in the room.
Compare/Contrast with his conversation with Chipp Zanuff later on.
Chipp is 1 Centimeter shorter than Sol, so technically Chipp is taller than either Ky or Zato. Gabriel seemed to be looking up at Chipp so he’s a tad shorter, but about the same height as Ky, so around 5'11".