and just to get the dumb stuff out of the way....
you have the monitor plugged into the GPU, not the mobo?
and when you removed the GPU and tried to get it going, you had the monitor plugged into the mobo and your mobo and CPU support onboard graphics?
you have the monitor plugged into the GPU, not the mobo?
and when you removed the GPU and tried to get it going, you had the monitor plugged into the mobo and your mobo and CPU support onboard graphics?