considering how little it has been used it is certainly very disappointing!
While the number of charge cycles matters a lot, a Li-ion battery keeps aging somewhat, even when not used. From what I have read, if kept nearly full for a long time (weeks), it degrades faster, which is why it is recommended to charge a phone to about 50% before putting it into storage. Having a Li-ion battery go completely empty is apparently even worse. Phones will typically shut off a bit before that, but if then left like that for long, that remaining charge will leak away and that can damage the battery severely. Temperature and the quality of the individual battery also matters. What I'm trying to say is that it can be quite hard to know why a certain battery ages quicker than it should.
To the best of my knowledge, there is no way to directly to directly measure the charge level of a Li-ion battery (its voltage only gives a hint). By trying to guess how the battery ages and somehow measuring the energy flowing in and out, a phone can indirectly track the approximate charge level, but small errors adds up over time. Only when the phone gets fully charged, or runs until it shuts off will the charge controller get any authoritative information on the actual state of the battery.
If your device stops charging at weird values and suddenly has begun losing charge even when connected to a charger, I would suspect your immediate problem is that the charge controller inside the device has lost track, and has locked itself into a kind of "safe mode" where it drains the battery until the device shuts down, to make sure it doesn't accidentally overcharge the battery. While I haven't experienced it with my Gemini, it has happened to at least one other device I have.
I would try keeping the Gemini on until it shuts down and then, much like nitrem suggested, connecting it to a PC/laptop USB (or other a non-fast charger) over night and see if anything useful happens. If the battery is very low, seemingly nothing might happen for a while, when connecting the charger. Some of my old Nokia phones could need half an hour or so to even light up the charging LED, if the battery was very low to begin with.
I have read various suggestions about (trying to) charge a phone to 100% and run it until it shuts off a few times to "re-calibrate" the charge controller. For me, running it until it shuts off once and then leave it on the charger for a few more hours, when it says it is full, has been sufficient, provided the battery wasn't actually dead for real, but given the sudden onset of your problems, I think your main problem lies elsewhere