I don't think that when you are 2/3 god and are able to resist the sum of entire humanities evil and come out just a bit crankier you need any helmet, I bet my good eye on it that he could swat away any telepathic attack like a fly over his head.
Presumably he is in full servant mode instead of the turned human by the grail one, so his armor is actually made of prana imitating metal rather then actual metal. Well about the Ea itself, I don't think Onslaught would even consider it a treat till it's not activated and when it is activated the world/space/time distortion effect might prevent any manipulation of said sword.
I don't think Gil would win in the end, not because he is not capable of winning, if he used every sheep trick in his arsenal and every treasure he can he probably would, but his ego would be his downfall, it always is.
If it come at worst, he could use Ea's final function and end the fight with a draw.