I hardly think that explanation is valid since she fades away all the other times.
And Gil didn't ask the Grail for anything, he was able to stay in the world because he was splashed with its contents, from what I remember it was an inadvertent splashing.
I could be wrong since I haven't read Fate/zero, but that seemed to be the case in F/SN.
If master's have enough prana, they should be able to keep Servants in this world, that's my view on this.