It could go eighter way if Gai gets cought by the raikage he is dead raikage can just liger bomb him but if A isnt at his top speed he could get ko or killed by the afternoon tiger. I think A wins he is physically the secound strongest ninja after Tsunade, he is so fast that he was able to dodge Amaterasu and the MS wasnt able to follow his movement and even if Gai uses the 8 gats he wount be able to keet up with A, if he is in that form for to long his chakra will draine fast and then A can finish him off.
Well, eight gate Gai wasn't faster than Tobi,was he? Raikage's speed his somewhere in Minato's league.. He's the second fastest ninja after all.. And there's also the thing. If Gai tries to go after him directly / with insufficient gates, he could get himself Ko'd.
A is the third fastest ninja after Minata and Naruto.