Probably the GPU
However, if you haven't done so already, I'd recommend taking out the GPU, cleaning the contacts carefully with some isopropyl (high % rubbing) alcohol and reseating in the PCIe slot. Also, if your board has a second PCIe slot, I would try it in there.
There's a small chance it could be ram (so you can try reseating that too and trying with one stick at a time) or PSU (If for some reason the PCIe connector isn't providing adequate current/voltage) but unlikely. Edit: And I guess it could always be the motherboard -as in failed PCIe slot...
The best would be if you had another system to try the GPU in to rule out for sure.