I have a gainward gt that I've flashed several times successfully, so I haven't ever had to do it myself, but if you can find a pci video card (or buy one for the cost of a 6-pack) you should be able to start the computer with video so you can reflash the gt.
Edit: To clarify
1. Remove GT
2. Install PCI video card
3. Boot to BIOS, set to init PCI video first
4. Shut down
5. Reinstall GT, leave PCI card in
6. Boot to Flash disk/cd, reflash card.
7. Reboot to BIOS set back to AGP
8. Remove PCI card.
9. Give it a try.