Would it be possible to retroactively generate the gem ores in a pre-made or already explored world?

Nope sorry, that is not how the game works. :(

Not a great solution, but I think the Chunky trim command can be used to delete some chunks from your world, effectively 'unexploring' it. Then when you visit them again, I think they could generate again. Not a big deal if you have a house and don't care about anything outside of that, but if you've built in multiple areas and want to keep that then basically no.