you'll need to flash the video BIOS to that of the GT
use a program called NVFLASH.EXE, put it on a floppy, with the BIOS of a GT, and BACKUP YOUR ORIGINAL 6600 BIOS, then flash it with the GT's bios
if it works, then great!
if it doesn't work, create an autoexec.bat with the command to flash the BIOS back to the original one. then put the floppy in and boot. once you know it's finished, reboot