Gaius Baltar cannot be a Cylon. We know exactly how many models of Cylon there are: 13. And they're all accounted for.
The eight we see most often: respectively John, Leoben, D'Anna, Simon, Aaron Doral, Number Six (i.e. Shelley and Gina), Daniel (who was boxed before the events of BSG), and Boomer/Athena; the "Final Five", survivors of the original Earth: Anders, Foster, Ellen and Saul Tigh, and Tyrol.
Aside: The note that Adama gets at the end of the miniseries says there are twelve models, but that's because it doesn't include Daniel, who is for all intents and purposes no longer a model of Cylon. We never see Daniel, but after
becomes aware of her Cylon-ness, she remembers him. And at that point she would have recognized Gaius as being a Number Seven if that were the case.