I used to use my Gstar with a 10"Dob but I was only hand guiding.
Needless to say I didn't have much success, but I could see the possibilities.
But mount it on an eq platform and you'll be able to produce some quite excellent images.
The FOV is quite small and if you want to use a focal reducer, the ota would need to be cut down about 40mm so you achieve focus.
But you have the 120mm, you can use that if you want a wider fov.
Have you joined the Gstar Users Group? Have a look at some of the images that these guys put out. Awesome stuff.
http://tech.groups.yahoo.com/group/gstar-users/