Make screen capture (Command + Shift + 3).
If the GPU cause this issue, then this shadow should also captured in the picture.
If the capture is fine, then it means the GPU can render the image correctly. But something wrong during display it. In this case, can be the card, can be the port, can be the cable, can be the monitor.
Anyway, for this kind of question, it will be so much easier for us to help if you can provide pictures.