It would seem that Gemini would do well to promote a companion device. This can be done in a few ways. For starters, even those who want/need a Gemini may not want to carry it all the time - so they will still have a conventional smartphone that is more convenient for phone calls and for taking pictures.
The other way round is to have a device that displays caller ID... smart watches aren't the only option here. Sony make a small device that has a display and call and media controls buttons that operates as a standalone Bluetooth earpiece or, since it has a 3.5mm socket, clips to your shirt and uses a standard wired headset.
If we abandon the Gemini and thoughts of full mobile Linux, another option would be the snap-on Qwerty keyboard for Motorola Moto Mod phones, but no full reviews of it are available yet. Early hands-on reviews suggest that it is not without flaws - but then said reviewers didn't have time to get used to it as the Reg's reviewer of the Gemini. This type of approach has the benefit of allowing the keyboard and phone to be upgraded / swapped out independently of each other.
If I were designing a mobile typing experience, I would have a standalone keyboard that works with any phone, and I would use the weight and bulk of a li-ion power bank behind the phone to keep it stable. A bit bulky, sure, but no bulkier than additional batteries are anyway. It would also divorce the screens longest dimension from constraining the length of the keyboard.
The whole issue of running full Linux on a mobile one is not really technical, but rather getting drivers not blobs out of ODMs.