Originally Posted by nico9
I don't want to develop a relationship with Z before she knows about what kind of relationship I have with G because it would not be fair for her to hide that. .
You've answered your own question. If you want to invest real emotion in Z and want Z to invest in you, tell her now. To let her fall for you first and then spring it on her is cruel in my opinion. We're still in a world where exclusivity is pretty much assumed when people form deeper relationship. Omitting information is as bad as lying in some cases. This is one of them.
Good luck, and hopefully she is just as open as you are. Maybe she has never had a boyfriend before because she doesn't want to be tied down herself. You could be just what she is looking for.