The Gold is very noticeably brighter, with more sparkle and very "live". The G12-65 is warmer and has a smoother top end. Both are great speakers but very different from one another. Its apples and oranges for me but I would be very curious to know how they would combine in a 2 X 12" cab. They are close in efficiency and watt ratings.
It really depends on the amp for what I would recommend. In my Vox AC30CC1 the Alnico Gold is perfect, with other amps the results have been more mixed from what I have read on here. Only recently actually have I seen alot of bad Alnico Gold reviews. Keep in mind for choosing speakers, the cab, amp used, and personal preference make a big difference.
I have a Weber G1265, its a nice speaker, but I miss the highs a bit with it. Very nice smooth midrange. I haven't compared the Alnico Gold with the same amp and cab as the Weber G1265 to make a real good direct comparison. And I have the Weber, not the Celestion G12-65 or the reissue.
I've had both, but not at the same time. I just sould a gold and bought a G12-65.
Gold - Chimey, 3D, thick & slightly congested lower mids, Alnico softening/compression at higher levels. More bass than Blue. Full low end but not as tight as G12-65. I sold it because of the mid congestion.
G12-65 - Balanced (slighty nasal but not broken in yet), smoother top end, more open in lower mids, not as much 3d sound. Tighter bass than Gold.