Sorry to revive an old thread, but I to was looking for such an inexpensive GCN 1.2 video card with no luck. I found this chart with which rebrands matched to each architecture. It is for OS X compatibility, but shows all the cards. It seems HD 7000 series starts GCN.
AMD5000Controller.kext
[TH]Graphics Card[/TH] [TH]OOB[/TH] [TH]min. OSX[/TH] [TH]DevID[/TH] [TH]Reference Port Layout[/TH] [TH]Framebuffer[/TH] [TH]Known Issues[/TH]
HD 5770 Yes 10.6.8 0x68B8 2x DVI, HDMI, DP Vervet none
HD 5770 Mac Edition Yes 10.6.8 0x68B8 2x mDP, DVI Hoolock none
HD 5850 Yes 10.6.8 0x6899 2x DVI, HDMI, DP Uakari none
HD 5870 Yes 10.6.8 0x6898 2x DVI, HDMI, DP Uakari none
HD 5870 Mac Edition Yes 10.6.8 0x6898 2x mDP, DVI Langur none
HD 5870 Eyefinity 6 Yes 10.6.8 0x6898 6x mDP, DVI Zonalis unconfirmed
AMD6000Controller.kext
[TH]Graphics Card[/TH] [TH]OOB[/TH] [TH]min. OSX[/TH] [TH]DevID[/TH] [TH]Reference Port Layout[/TH] [TH]Framebuffer[/TH] [TH]Known Issues[/TH]
HD 6790 No ? 0x673E ? ? DevID missing
HD 6850 No ? 0x6739 ? ? DevID missing
HD 6870 Yes ? 0x6738 2x mDP, HDMI, 2x DVI Duckweed 2nd DVI non-functional
HD 69x0 No ? - - - very buggy, not usable at all
AMD7000Controller.kext (GCN 1.0 architecture)
[TH]Graphics Card[/TH] [TH]OOB[/TH] [TH]min. OSX[/TH] [TH]DevID[/TH] [TH]Reference Port Layout[/TH] [TH]Framebuffer[/TH] [TH]Known Issues[/TH]
HD 7750 Yes 10.8.3 0x683F ? ? Black screen issue since 10.10.3
HD 7770 / R7 250X Yes 10.8.3 0x683D 2x mDP, HDMI, DVI Dashimaki Black screen issue since 10.10.3
HD 7850 Yes 10.8.3 0x6819 ? ? none
HD 7870 / R9 270X Yes 10.8.3 0x6810 2x mDP, HDMI, DVI Futomaki none
R9 270 / R7 370 No 10.8.3 0x6811 ? ? DevID missing
HD 7950 / R9 280 Yes 10.8.3 0x679A 2x mDP, HDMI, DVI Hamachi none
HD 7970 / R9 280X Yes 10.8.3 0x6798 2x mDP, HDMI, DVI Hamachi none
AMD8000Controller.kext (GCN 1.1 architecture)
[TH]Graphics Card[/TH] [TH]OOB[/TH] [TH]min. OSX[/TH] [TH]DevID[/TH] [TH]Reference Port Layout[/TH] [TH]Framebuffer[/TH] [TH]Known Issues[/TH]
HD 7790 / R7 260 Yes 10.10? 0x665C none none Buggy multi screen support, buggy DP
R7 260X / R7 360 No 10.10? 0x6658 none none
R9 290 / R9 390 No 10.10? 0x67B1 none none
R9 290X / R9 390X Yes 10.10? 0x67B0 none none
AMD9000Controller.kext (GCN 1.2 architecture)
[TH]Graphics Card[/TH] [TH]OOB[/TH] [TH]min. OSX[/TH] [TH]DevID[/TH] [TH]Reference Port Layout[/TH] [TH]Framebuffer[/TH] [TH]Known Issues[/TH]
R9 285 / R9 380 Yes 10.10 0x6939 none none Works in 10.10, Black screen in 10.11
R9 380X No 10.10 0x6938 none none "
Fury / Fury X No ? ? ? ? No drivers yet
From what I can tell, none have full DX12 support, but AMD seems to have the best as they have the Asynchonous mode and Nvidia doesn't yet. Nvidia has more graphics options than AMD, but Asynchonous mode give a huge boost in performance.
Anyway, I dug out my old HD 7750 for GCN 1.0. I think I'll wait it out and save up for the next gen cards which, I assume, will have full DX12 support.