Gai can't beat A in neither strength or speed. It's a bad match-up for him, and I don't even see his hidoura hitting A.
Gaara also has a fairly strong defense, and Gai wont have much defense against ninjutsu used by Mei or Onoki. Even if his Hidoura one shots most, the chances for him being one shot before are rather high.
Don't overestimate Gai. U_U People keep thinking his speed is comparable to A and Naruto, it isn't.. Look how beat up he got in the last fight with the wood dragon. He hadn't speed to dodge anything in that battle.
Tsunade he probably beats, but then again she can probably take many punches and regenrate, and if Gai goes 7th gate too fast, he will lose, since Tsunade is durable enough till 7th gate after effects step in..
And 8th gate, well nothing can be said about it so far, since we haven't seen it yet..