The Broadkaster's pickguard is shaped so that there is about 80 millimeters of space between the pickups. The G5123B has pickups that are about 90 millimeters apart. You could use a Broadkaster pickguard, but there would be about a 1 centimeter gap between the pickup and the cutout on the pickguard.
Gretsch does not make a pickguard designed for the G5123B. You can make your own as I did (check out the second link in the post above), or have one made for you. I used the pickguard from a G5120 guitar as a template, and simply modified the template to accomodate the G5123B's unique string spacing: