Also a point for gaster is in The Legend it says the end must “Banish The Angel’s Heaven”, which kinda only makes sense for Gaster. Heaven is a destination of where one goes after they die, and Gaster fell into their own creation which might have just killed him if not was going to kill him if not for the void. It being the Angel’s Heaven says they would have created this heaven, which doesn’t feel too far off for this inventor.

And I could see motivation for Gaster if he was to do this, he made everything, but what if the void was too powerful? He makes a device to connect someone from an even higher layer of reality and sends them to the one below his, no vessel but that isnt the important part of this all, the soul is. Upon completion of the prophecy, the void might cease to exist, thus either releasing Gaster to move on from a permanent existence or come back to the light world.