2.2.2: What was up with Gawyn during the Tower Coup?
It's been suggested that he was involuntarily bonded to a Black or to a Forsaken. His eyes were glazed and he was not necessarily in control of himself [TSR: 47, The Truth of a Viewing, 539-541]. Alviarin says with some confidence that "Gawyn will be brought under control". [TFOH: Prologue, The First Sparks Fall, 17]
OTOH, it is perfectly reasonable to assume that he was in full control of his faculties during the coup. By that time, he had developed a good and solid hatred for SS, due to Elayne and Egwene's mysterious disappearance. Furthermore, in his POV scenes in LOC and ACOS, he never thinks anything that would give the idea that he's bonded to anything. Plus, he agrees to be Eg's Warder; he couldn't do that if he was already bonded. (Since he'd trained with the Warders, it's not reasonable to think that he wouldn't know a Warder bond when he felt one.)
From his behavior in LOC, it seems pretty apparent that he acted under his own will during the coup, motivated by grief and anxiety over Elayne and Egwene. He develops a similar fierce hatred for Rand due to the rumors that Rand killed Morgase.