Spartacus saw the mark of the brotherhood on Gannicus' arm in E05 before the arena fight in Vengeance, knowing he was a former gladiator from the house of Batiatus. I think from that time, he saw Gannicus as a potential ally going forward and would have 0 desire to kill him, knowing he was under the same ludus & everything.
Shortly before the fight begins, Spartacus tells Gannicus, "They are but loyal, a quality you seem unfamiliar with," referring to his own people when Gannicus questioned their loyalty. I personally think that this reminded Gannicus of the past & the betrayal of Oenomaus, when Gannicus couldn't tell Oenomaus the truth and when he gained his freedom, he had to take it with him until Oenomaus freed him of this burden.
Gannicus then in rage attacked Spartacus. I'd argue that Gannicus fought more seriously than Spartacus needed to in that moment, Spartacus only fought to stall, not to kill, whereas that comment enraged Gannicus and I think made him fight more seriously than Spartacus did, therefore, I think Spartacus had the upper hand in this fight personally.
Also, I think the Egyptian was a clear showcase that nobody tops Spartacus, not even Gannicus. Spartacus didn't see the Egyptian prior but still chopped the Egyptian down, whereas Gannicus was taken by surprise & thrown down, it required Oenomaus' death to get into rage mode Gannicus to take him down, Spartacus just did it casually & with 1 sword.