Any of the Olympus flashguns work on the G1, the FL36, 36R, or FL50, 50R. They will tend to be more easily available and cheaper than the Panasonic versions. They are fully functional with the G1. The Metz 36 with Olympus mount also works, although this doesn't have a swivel head, only up and down.
I have an Olympus FL50 and an FL36, and the FL50 is a bit heavy for the G1, but the FL36 is perfectly balanced on the flash shoe.