STICKYTIME! http://www.findmall.com/read.php?80,1332879
The GB/G2 is designed specifically for DD searchcoils from the "Greek" (a.k.a. "Frat Bros.") series. That's the 11 incher and the 5 incher. Maybe there'll be others in the future, but for now, those two searchcoils are it.
Some customers have tried Fratbros concentrics on their GB/G2. Results have been mixed all the way from seemingly perfect results to no function at all (continuous overload). Nobody should be representing that "the Greek series coils work on the GB/G2". The DD's do. An individual concentric may or may not, or may have degraded performance that either you don't notice or that you do notice but decide you can live with.
The F75/F70 coil series pins out the same, but is electrically different. Several people have reported plugging searchcoils from that series into the GB/G2 and having the thing seem to work. Whatever "work" means in this context: performance was probably degraded but in ways the customer wasn't aware of.
T2 searchcoils are wired different and flat out won't work at all on anything other than a T2.
SUMMARIZING: If you buy a searchcoil to use on a GB/G2 that's not specified for the GB/G2, maybe you'll like it or maybe you won't. The risk is yours. No complaining to the manufacturer, not even to the dealer unless the dealer misrepresented the merchandise.
--Dave J.