I didn't buy the 1W but the 3W - it worked a bit...you take the metal ring out of the recess halfway along the drop in body and,sort of, stretch it into a springy looking thing (Half a spring really) bung it onto the end of the drop in and cram that into your G2, then screw on the head and hope that it works!(descriptions are on the flashlightreviews.com website for the BOG drop in).
Sometimes it worked, sometimes not. I have heard that in general it's a bit hit and miss, depending on your G2.
However it does work on my C2 and even my LEDWAVE X1!! And it's a very good drop in, just a bit frustrating.
I'm sure it could work better with a bigger spring, like on the bulbs from Surefire, or even with a blob of solder.
Persevere!!...then go buy an INOVA T2 for about the same brightness and longer runtimes!!!