I have a XT1900GT Mac PPC radeon card.
Are you sure that this card is Powermac G5 version?
As I know, there was ATI Radeon X1900 G5 Mac Edition
for out late 2005 Powermacs. I.e. PCIe G5 dual core and Quad.
I have exactly this card and it works.
Original MacPro ( Intel ) had ATI Radeon X1900 XT.
I am not found sign "1900 GT" on powermac cards. Yes, G5 edition have the same graphics chip like GT, but different bios.
Of course, it can be labelled different ways, but I recommend double check if you have real Powermac G5 version.
And of course, If you have original Powermac NVidia in first slot, you can use PC version of X1900 GT card for Morphos and linux.[ Edited by sailor 24.01.2023 - 07:47 ]
AmigaOS3: Amiga 1200
AmigaOS4: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad