So, how did you test with no OS?
When you tested, did you connect the monitor to the mobo, the GPU, or both?
What`s your motherboard model. I wanna check if you can disable the integrated graphics in bios.
And another thing - Is this a new build, or an older PC? Did you have a GPU in there before?
I think that in order to fix this, you should do something like this:
1. Connect the monitor to the motherboard, and plug in the GPU, and connect the power cables.
2. Power up the PC, you should be able to get an output.
3. Install the GPU drivers from the manufacturer website.
4. Restart PC, and go directly into BIOS.
5. Disable integrated graphics. Save, and shut down.
6. Connect monitor to GPU - you should now have a video output.