When to drop the poly bomb
I had a relationship with my girlfriend G for one year but we were not living together. We were in the same city at the beginning but after a few months, we both moved in different countries (to follow work and studying opportunities) and with the long distance we could meet each other only every 1 or 2 months. We were both new to polyamory but we actually talked at the beginning of the relationship about open relationships and we talked openly about our attraction to other people when it happens. She had a crush to another guy 6 months ago, their relationship unfortunately didn't work (that guy didn't accept that she was still feeling something for me) but this reinforced the idea that polyamory is good both for G and for me. After 1 year, our relationship changed from "boyfriend-girlfriend open to the polyamory idea" to "two close friends" who enjoy spending holidays together, including having sex together when we meet but who are available for other relationships and we don't have any project to live together in the coming years.
That is my story with G but my question is actually not about G. I met someone else (her name is Z) three weeks ago, we were going out with other friends and we had a date together a few days ago. I didn't talk about polyamory yet and I don't know how and when to do that. 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. I will meet G again (mid-October) and I'm not giving her up. I believe Z didn't hear about polyamory before. Z also never had a boyfriend before so I would be her first boyfriend if it goes well. I am also not a "serial monogamist" because G was my first girlfriend. I am not openly poly in general, only a few friends know (and G knows everything of course). I fear that Z would not accept the situation, that's why I didn't dare to tell her a few days ago. But I want to tell her soon, so I'd be happy to read any wise comments.