First off, I am decidedly Canon biased, but only because Canon is by far and away the best thing out there. Looking over the GX-100 specs, DPR doesn't say whether or not it accepts accessory attachments. We know the Canon G-9 does. Just add the snap-in threaded barrel, and you can attach Hoya close-up lenses. I've had good experiences with these. The G-9 also accepts any Canon speedlight.
The GX-100 is unique, in that the lens goes down to 24mm, and is relatively fast. The G-9 is F/2.8~4.8, compared to F/2.5~4.4 for the Ricoh. DPR says that there is some corner softness with the G-9 at the long end in macro, but distortion is very low. This tells me that all things considered, the G-9 is the better, and maybe the best camera in class for macro, esp. with the accessory lenses.
If you do buy a Canon flash, it is compatible with any Canon DSLR, and it shares the same battery with the XTi. The G-9 is certainly the most complex compact cam, and fully able to back up your DSLR, if you ever opt to get one.