There are alot of things in a relationship that can have a workable compromise. Except religion and kids. These are not compromisable. Personally, I don't believe in my SO converting "for my sake". Religion is something one needs to discover and believe for themselves, and conversion is not something you do for someone else's sake. So I do think your gf is going about it wrong, but she is at least certain about what she wants in her future.

You can look at it as she's giving you an ultimatum, but maybe she isn't. She's just telling you the reality of what she sees her life goal wrt marriage is. That is she doesn't want an unbelieving spouse. She is giving you an out if that's not what's aligned with you before you guys make further plans for the future/propose, etc. Kind of like in the case of wanting kids vs being childfree. Your gf knows what she wants or do not want.

So, carefully consider on your end if Catholicism is for you. If not, don't waste anymore of each other's time.