I think at this point you are at a standstill until you can check the GPU. The problem could be either the GPU or the motherboard. If you happen to have a two pin piezo speaker, you can hook that up to see if the system is giving you beep codes.
Another question - the P500A allows a vertical GPU mount. Do you have the GPU installed directly in the motherboard PCIe slot, or is it installed vertically in the case with a riser card/cable to the motherboard PCIe slot? If you have it installed in the case and not in the motherboard, try plugging it directly into the motherboard while troubleshooting. Risers can be a major pain to deal with.
Since you've built your first system, you'll inevitably do this again in the future so a piece of free advice.
When first assembling a new system, to save yourself a lot of misery you need to test your hardware before installing it in the case. Get a piece of cardboard large enough to set the motherboard on. Place the board on it, install the CPU/heatsink, a single DRAM module, and GPU (if your CPU doesn't have onboard graphics). Hook up all the power supply cables to the motherboard/GPU as required , hook up the video cable to the monitor, then use a screwdriver to short the power switch pins to start the system. If the hardware is good, you should at least see a BIOS boot screen (with a keyboard/mouse plugged in, you should be able to access the BIOS screens).
The advantage of doing it this way is so you absolutely know your hardware works before installing it in the case. That way, if it doesn't work when installed you know the issue is probably mounting-related (such as a stray motherboard mounting stud causing a grounding issue in the case, etc).