When is the right time to tell GG you're a crossdresser

[Pat]

I underdress all the time, even for work and dress at home, but officially I'm still in the closet and dressing is not part of my social life.

So, it seems likely you'd want to disclose before that top layer of clothing comes off, right? I don't know what the culture is like in Slovenia, but in the US that's usually pretty early in the relationship. No matter which way she reacts to the revelation, you have to be prepared that the information may travel -- so consider who else you might want to disclose to before it gets out or at least have a plan for dealing with it.