For better or for worse (and whatever Google's fine print says), most people understand the term ‘Android’ to mean the full Google stack, and not just the open-source parts (if they're even aware of the distinction).
Yes, there should probably be a separate name for the latter, but there doesn't yet seem to be a widely-understood one. So if you want to avoid confusion, I can't suggest anything better than ‘Android’ (or, if you want to be really clear, ‘full Android’) for the former, and a phrase like ‘a free Android-based OS’ (or ‘an open-source OS based on the free parts of Android’) for the latter.
(And while I have a lot of sympathy for the idea of sticking to open-source software, please remember that the vast majority of people are happy with the full Google stack, and so will find the Gemini's telephony all working fine. It would be a shame if misunderstanding your review put people off getting a Gemini unnecessarily.)
Also, FWIW, it is possible to use a Gemini as your only mobile phone; I've been doing it for months. I don't take very many voice calls, but I find it fine for those I do: it's no bigger or uglier than many other full-size phones, and the sound quality is plenty good enough. I also find it far better than any other phone for messaging, even while standing or walking: as a long-time Psion 5mx user, I'm very used to thumb-typing. Making calls is a little fiddly, but for me that's greatly outweighed by the convenience of having everything in one device, and no longer having to fill my trouser pockets with different devices. — Of course, other people will have different preferences, but I think it does the Gemini a disservice to say that you can't use it as your only phone.
Anyway, I've ordered some black eGrips. It could take several weeks for them to reach me in the UK, though, so don't hold your breath!