Well the UAC-10 is what, 13 tons? The IS Gauss is 15. But, you've also got to account for ammunition. The autocannon gets 10 shots per ton, so you'll want at least 2 tons. Now you're at 15 tons. That gives you 10 rounds of double-tap fire, with an average of about 15 points of damage per round (and a chance to jam). For that, you generate 8 heat. You also get PPC range.
The Gauss is 15 tons to start, and while you'll want more ammo, a single ton will give you 8 rounds of fire. So for one extra ton, you get 2 rounds less fire, the same average damage (just a hair higher, actually), better range, and 7 less heat. You also get 15 point hits instead of potentially 2 10-pointers.
The more ammo you add (say 3 tons for the Ultra and 2 for the Gauss), the more the ammo advantage swings in the Gauss' favor. Now, it's true that at high to-hit numbers, the Ultra can fire single-fire and conserve ammunition. But I think they're close enough all around that people will normally take that next step up and choose the Gauss.