No way to make that alliance work - choosing gortash automatically puts you at odds with karlach's entire storyline. Makes sense when you know he's the one who sold her to zariel, leading to years of torture and engine experiments. When you reach the coronation, you'll need some solid persuasion checks just to keep her from attacking him on sight.

Going with gortash means betraying everything karlach stands for, especially since he took her engine tech to build his steel watch army. The whole situation's designed as a clear choice - either help karlach get her revenge, or watch her literally burn up from holding back that fury. There's no middle ground here, and your approval with her tanks permanently if you side with him.

Might be worth noting that if you're planning the gortash route, be ready for some serious fallout with karlach, including her possibly leaving the party altogether. The game really drives home that whole deal with the devil angle here.