Your G1 is micro 4/3 mount, that means any micro 4/3 lens will fit on it.
Unfortunately there aren't many choices for macro on micro 4/3 mount, but the panasonic 45mm seems to get good reviews.
I would steer clear of most adaptors for now, the cheap ones you see on ebay are purely mechanical so you have no automated functions on the lens. This really throws you into the deep end, and while some would say that's a good thing (forces you to learn), macro is frustrating at the best of times and you're more likely to be able to get into it with a lens that's easier to use.
However, I believe olympus and panasonic make micro 4/3 to 4/3 adaptors which keep electronic control (google panasonic DMW-MA1E or olympus MMF-2), though they are quite expensive. But this would allow you to mount any 4/3 lens (there are a lot more macro lenses available in 4/3) onto your G1 and keep at least some electronic functions (though the details I've seen on what electronic functions they retain are a bit sketchy). Then you could get something like the excellent sigma 105mm macro in 4/3 mount and it could still work out cheaper than the panasonic 45mm (plus it also widens your lens selection for any future requirements.
The advantage of something like a 105mm with adaptor would be that although it's bigger and heavier you get more working distance from your subject and the longer focal length creates more diffuse, pleasing backgrounds. But the panasonic 45mm is nice and compact and light.