[quote=@Flamelord]
<Snipped quote by Suiko>

This might just be me, but I'm not seeing what Suiko did that earns her Gate of Babylon as a Phantasm. That's pretty specifically Gilgamesh's thing.
[/quote]
It technically is possible to have Gate of Babylon because Gilgamesh does reveal that he has many keys to open his dimensional vault except he threw all of them away. Someone could very well have found one of these keys, but even then it's stated that if you do find such a key you only get to access your own personal vault rather than Gilgamesh's.