G12-80's are fairly rare speakers, only having been made for 3 or 4 years I believe ('78 to '82?). If the cab is in decent shape and the price is reasonable, I'd grab it just to get those speakers.
I think the only difference between the guitar and bass cabs was the speaker cones on the G12-80's. The "bass" version had the 444 bass cones, whereas the "guitar" version had the 1777 cones. Regardless, the 444 cones work great for guitar. If it has the 444 cones, the speakers will essentially have the same bass response as a Vintage 30, with the mids and top end of a modern day Classic Lead 80. Can't complain about those characteristics.
