Might just work! The "melt their gems for real bitcoin" part means that whatever legal entity holds it will need a whack-load of special licenses, but there are fintech labs out there that can provide regulatory cover without a big investment, though that'd likely mean another entity.
If the gems are used like in a game that wouldn't be a concern (as in if the gems can't be cashed out except to buy the feeling of fun, as part of some gamification element).