I think that if Gai opened all of his gates he would be a pretty powerful oponent against Orochimaru.. But there's no way that he'd actually win. Even if he had Lee Neji and Tenten backing him up.. Orochimaru could just use Edo Tensei or his weird snake mode if he was really in trouble..
Oro would win He is a sanin and knows a lot of jutsu and is informative and has more experience plus Gai tm is mostly taijutsu whch does not work well with Oro