I honestly believe that Gamzee’s relationship with Terezi was completely manufactured by Aranea.
We first meet Aranea during her conversation with Terezi. At this point, we learn a lot of things about her: she is a godtier Sylph of Light, she is aware that Terezi is a Seer of Mind, she has been watching Alternia for a long time, she respects her alternate Alternian self, and she feels driven to heal others. Her offer to heal Terezi’s eyes seems innocent enough at this point in time, and she seems much different than her Alternia counterpart.
Later, during the Meenah quest walkaround, we notice that something’s up with Terezi. She isn’t showing her eyes, and she seems put off by the appearance of the dancestors. We also learn that she and Gamzee are in engaging in a kismesissitude.
Then, we get Terezi’s confession about Gamzee and this:
The fandom likes to jump on Gamzee for his abusive relationship with Terezi, but the question should be asked: how long has Aranea been in control? And why is she in control at all?