well im nt exactly sure but from wht i understood i think the gate is only able to open the gate for the souls to come in our world full stop! However, i think in Lia's case she happens to b able to do both, open the gate And b the angel meaning she can call Samael and bring an end to the whole thing. As mentioned in the book, this double ability may happen again the future maybe another century or so but nobody can tell. Being the gate and the Angel at the same time doesnt always happen. Hope that made things a bit more clear for you.
That's exactly right, Michelle! Previous gates have been able to usher in Souls, which Samael wants because the more Souls in our world if and when he arrives, the better. But only Lia is the Angel, and only she can usher in Samael himself. Should she succeed in NOT allowing him entry, there is the possibility a future Sister will be marked as an Angel unless Lia closes the Gate for good by ending the prophecy, which is what she's attempting to do.
Stay tuned for book three! I promise all your questions will be answered.