Oh man this is a tricky one because karlach really has it out for gortash, and I totally get why. I tried working around this in my playthrough but there's honestly no perfect solution here. You can try to play both sides for a bit, like agreeing with gortash to get some tactical advantages with the steel watch stuff, but karlach's gonna be pretty upset if she finds out.

From what I've seen, you'll eventually have to pick a side since karlach's whole story is basically about getting justice from this guy who sold her out to zariel. If you're trying to keep karlach happy, which I'd recommend since she's such a great companion, you're gonna want to steer clear of helping gortash too much. Even if you leave her at camp while doing some gortash-related stuff, it eventually comes back around and causes problems.

Something interesting I noticed is that even if you try to work with gortash, he ends up betraying you anyway or gets taken out by the netherbrain, so there's not much point trying to stay on his good side. Plus, working with him too much locks you out of some really cool scenes with karlach later on, including some pretty emotional moments at the graveyard.

Bottom line, sometimes you just cant have it both ways in these situations, and this is definitely one of those times. Trust me, its better to just pick your side and stick with it.