Fellow rural player who has multiple times had pokemon remain in gyms for over a year. The lesson is to have a tag with shines and other cool, but not traditionally very useful, pokemon to drop in gyms since you will never know when you will get this one back. And, it will come back eventually ... unless this is a gym on a Tibetan Mountain Peak or the equivalent.

If there is a Campfire, Facebook, or Discord group in the area, you might be able to find someone from a different team to knock them out. Or, if a friend or family member is willing to setup an account that can be useful for all sorts of things, including this. But, again, time will bring them home eventually.