@maledentto30
Actually they answered this in one of the type moon ace volumes.
Gilgamesh has his clairvoyance np active at all times, its not something he has to invoke.
Sha Naqba Imuru is really strong for clairvoyance as well, up there with what Solomon and Merlin have.
This is including in Zero and Stay Night. The issue is that when he sees things which overpower him such as that he was to be swallowed by the mud he rejects it as nonsense and disregards them.
His pride is always his downfall because he doesn't believe he would ever be defeated in such a way so it can't possibly be true.
Here is the quote
TYPE-MOON Ace Volume 10, Narita:
"As Gilgamesh is also a fellow clairvoyance user, I asked about the state of Gilgamesh’s memories when he’s summoned. The answer I got was that Gilgamesh can see the possibilities of various parallel worlds if he so chooses, but the Throne adjusts the memories and knowledge of the Heroic Spirit according to the world they are summoned in, so he won’t be confused by having multiple sets of memories. I was told that even if he did use clairvoyance to see the possibility of a world where he was swallowed by the mud of the grail, Gilgamesh would reject that world line as being nonsense. "