Sirius + Rosier (+ Gideon)
Just because Gideon's life was more relaxed in the village (at least since the attacks) than it was in his normal life, didn't mean his instincts were waning. He was still very much an Order member, and although he wasn't so much fighting Death Eaters anymore, he was still ready to jump into any fray. So when he saw one forming at the masquerade ball, Gideon was a bit surprised but immediately kicked into gear. He told Amelia he'd be back and left her side to go take care of whatever brawl was starting. He had no idea who the masked person was, but Gideon saw Sirius punch the other man and moved faster.
Arriving at Sirius' side, he stood at an angle between Sirius and the masked person, not entirely blocking them from each other but enough to insinuate himself into the situation. "Mate, you really don't want to get into this right now." He hadn't seen what instigated the fight, but it wasn't worth it in the very public situation they were in.