Comment 5 for bug 806559

I agree that removing gdm is too dangerous. It would be nice to remove it, as upgraders really don't want two display managers lying around after upgrading. I'm guessing nothing will depend on it so an autoremove would get rid of it anyway?

So "don't ask on upgrade" is the best option in my opinion. It would be best if it did ask you if you installed it at a later point (e.g. KDE users don't want KDM automatically replaced without asking).