Karkoff at her most genuine would feel pretty deflated by Gangrene- he doesn't have very strong reactions to anything, and Karkoff absolutely thrives off of going back-and-forth with others. In a relationship (and even close friendships), she wants someone who can either keep up, put her in her place, or just isn't afraid to show off an equally strong personality. Karkoff jokes, and brags, and can easily talk herself into trouble... And that's just not the sort of person Gangrene hangs out with. He's certainly not got anything against it, it's just not his vibe.
Gangrene is just reserved, if not distant and borderline eerie at times. (Though the latter comes more from being a hemomancer, and from whatever his affliction is... Anyone close to Gangrene wouldn't think of him as "eerie.") Unlike Karkoff, he's almost always in his "professional mode," but, like Karkoff... It's not really all that different from his genuine self. But, Gangrene isn't opposed to being in a relationship, he's just... well, as said, he has a tendency to unsettle others for reasons beyond his control, really. Luckily, Karkoff wouldn't actually be one to judge him, and would probably give him plenty of respect for combatting the Shade. So, they wouldn't really be much if they were dating, or even as friends, but they could probably manage fine in a group setting together. Which, fittingly, is what Gangrene has with the druids, and what Karkoff has with her crew.
I actually think Karkoff might prefer to introduce Gangrene to someone a little more his speed... Someone equally passionate about their craft or hobby or what have you, and who's also a little quieter, just as non-judgmental, and can easily talk to most people that can't really "keep up" with Karkoff. But that's just speculation ;)
