Fate/Zero

[Buffalobiian]

Sure, Kiritsugu is someone special, being able to kill his inner representations of his wife and his daughter

You're making them sound like "illusions" David, while what they are are representations of the chances of having his wife and daughter back. He literally crushed the last one.

Since he didn't know about it, it could only be a deception...

Not necessarily. Gilgamesh admits himself that he owns more treasure than he knows. His conclusion is actually the opposite of yours: If it's a treasure that exists, then it must be mine already. He doesn't care for the grail since it's his, but he does care that other people are taking his property, so he'll defend it.

[Ryllharu]

Not necessarily. Gilgamesh admits himself that he owns more treasure than he knows. His conclusion is actually the opposite of yours: If it's a treasure that exists, then it must be mine already. He doesn't care for the grail since it's his, but he does care that other people are taking his property, so he'll defend it.

Wasn't there a scene where Kirei had to convince Gilgamesh that the Grail was worth having? Something new finally worth adding to his collection?

It wasn't a matter of making him realize it was already his, Gilgamesh seemed to understand that the Grail was created after he could have possibly ever owned it. He is a few millennia old, while the Grail is only a few hundreds of years old. Kirei had to convince Gilgamesh that it was something he should bother himself with.

The same way Gilgamesh spends time trying to convince Saber to become his wife. He knows she didn't exist when his collection was amassed.