When you see multiple geohashes on an event, it's actually kind of a hack that is necessary so that when zooming in a map a given event is visblible at various levels without too much extra work. But this wasn't anticipated when the g tag was added to the protocol.

The g tag might only be referenced in nip 52 but it was around before that; it never came from a NIP but was just kinda added at some point.

So I don't think it needs to be part of NIP 52. Probably a new NIP, but because it's just a tag storing data and doesn't prescribe any new behavior to the protocol, a NIP may not be necessary. Just start using it! 🫡