The jack plug with the 'extra' connection (i.e. tip, two bands, and collar) is for the mic and buttons. (Traditional earphones only have tip, one band, and collar.)
Most earphones seem to come in two models: one for Apple stuff, and one for Samsung and everything else — so I assumed that Apple uses those connections differently from everything else.
I guessed that the Gemini is of the latter type, and got a cheap pair that were advertised to work with Samsung. They seem to work fine with my Gemini while it's open: the volume up/down buttons work; the middle button works as play/pause, while pressing it twice or three times in succession acts as track forward/rewind, and holding it down activates the Voice Assistant.
(The buttons are ignored while the Gemini's closed; I'm hoping that's a firmware issue they will fix.)
If your iPhone-compatible earphones work just as well, then either the Gemini's clever enough to distinguish them, or there's less difference than I suspected!
