Full power Saber with Avalon vs Actually trying Gilgamesh, who wins?
Gilgamesh most likely. Saber has to come out of Avalon eventually, Gil can retreat a bit back, and then rain down on her when she does, and I wouldn't be surprised if Gil had a NP that let him access different dimensions.
As much as gilgamesh usually jobs he would likely win, I remember in reading somewhere he could even overpower Arcueid
He was, actually. That was the whole point of Saber not using Avalon the second the fight started, she needed to lure Gil into overcommitting to an attack and leaving himself open for a counter. So she trolled him into going "fuck this, you die now" and spinning Ea up to full power.
Full power Saber, like with Rin as her master or something? Would be like that except moreso.