I downloaded the manual for the Gigabyte motherboard you linked to above:
http://download.gigabyte.us/FileList/Manual/mb_manual_ga-p55-usb3_e.pdf
On page 7, at the top of the board, you'll see the ATX_12V_2X4 plug...(AKA 1 x 8-pin ATX 12V power connector)
On page 22, it says:
"1/2) ATX_12V_2X4/ATX (2x4 12V Power Connector and 2x12 Main Power Connector)
With the use of the power connector, the power supply can supply enough stable power to all the components on the motherboard. Before connecting the power connector, first make sure the power supply is turned off and all devices are properly installed. The power connector possesses a foolproof design. Connect the power supply cable to the power connector in the correct orientation. The 12V power connector mainly supplies power to the CPU. If the 12V power connector is not connected, the computer will not start.
• Use of a power supply providing a 2x4 12V power connector is recommended by the CPU manufacturer when using an Intel Extreme Edition CPU (130W)."
My understanding, (and I'm NOT very techy) is that you SHOULD still be able to POST without that plug, but it's recommended. Your PSU SHOULD have included it.
Your problem could be caused by several things...CPU not seated correctly, Memory not seated correctly, video card not seated correctly, bad sockets for CPU and/or RAM.
Unplug the box from the power source. Remove all your RAM, then install ONE stick of RAM in socket 3_1 or 3_3. Try to start the computer. If it starts, shut it down again, (unplug from the power source) and install the second stick in the other 3_ socket.
(see your manual for socket identification.) Try again.
If the computer doesn't start with ONE stick of RAM, I'd next check the CPU to make sure it's in the socket correctly.
I haven't built a computer in several years, so I'm more than a bit rusty in this...(but will be starting my own new build tomorrow or Sunday) Hopefully, one of the more experienced folks will read this and chime in with more suggestions.