What i meant by dead is in the sense of as janus says she isn't their mother, she is controlled by Lavos. And because of that when they defeat her(as well as Lavos) what gaspar probably means is that she isn't a loon anymore, since according to Schala she was actually a good person at one time and she's probably back to that so her being evil I don't see making much sense since
A.) She's good once again and
B.) Considering she's the almighty and all wise queen you'd think she'd have the intellegence to not make the same mistake again.
besides C.) Even if she lacked that intellect, she certainly wouldn't have anything that would allow her to become ultra powerful anymore. No more magic, no more technology, no more slaves, no more anything.
As for Gaspar knowing the stuff it is possible that since the other gurus are gurus because of their powers and his allows him to see into time. or maybe he can see anytime period from the End Of Time?
Lastly, obtain immortality I assume means that she's either thinking same age forever, only killing her can succeed in her death or more likely since she transforms and attacks I assume that it means something along the lines of living forever at the same age AND nothing can erase her from existence.
Somehow it has to relate to her never aging because how else can you explain her on the Black Omen in every time period never looking different?