I also second JayDDees recommendation about testing with a different GPU if available or testing the GPU in a different mainboard. But I wouldn't rule out power yet, these mainboards are weird af lol.
Cheers and good luck!
It's always risky to assume. Anyway it can be tested by using a riser and drawing power directly from the PSU.
If the PSU itself was the problem it wouldn't boot with the GPU inserted. But it doesn't hurt to try a different PSU
if one is available.
If it's not the GPU and not the PSU the only thing left is the the PCIe slot(s) on the motherboard.