I know for certain that the G-9100 and GW-9100 both use the same lug and bezel design, so their bands are interchangeable. Also, the GW-9200 Riseman uses the same band mounting design, so bands from that model are compatible as well. This was important because the GW-9100 band is generally considered not as comfortable as the Riseman version, as well as having a single tang buckle instead of double-tang. A number of WUS/F17 members have bought the Riseman band and installed on their Gulfman (me included).
CASIO wised up and when they released the GW-9110, the band looks very much like the Riseman style. However, I'm not totally sure about compatibility with the earlier Gulfman models. Can someone confirm that the GW-9110 band fits on the GW-9100/G-9100?