If you have your little MB speaker attached, you should be able to hear some beeps. 1 beep would mean all is well, more than that and you'd have to look up the beep codes for your MB. No beeps is weird (unless you don't have a MB speaker).
Try breadboarding the system outside the case. When you remove the board, check that there isn't an errant standoff under the board where there shouldn't be one. Test the system on an insulated surface with JUST the video card, monitor, KB attached (to a USB2.0 port). You can start the system by momentarily shorting the 2 pins that the case's power button would connect to. You should get a display (and be able to enter BIOS). If not, I would suspect the board is defective. If you have access to another gxf card, swap it out just to be sure it isn't the card that is defective. Or test the card in another machine.