hi there,
the only things that I can think of are:
Switch Polarity:
When you connect the PWR lead to the motherboard header are you getting the polarity right, + to + and - to - ? Try turning the connector round and see if that makes a difference.
Short Circuit:
When you are mounting the Gigabyte motherboard have you made sure that the standoff mounts are in the correct position? If you are mounting a larger board where before the board was possibly smaller or vice versa the standoff's will need to be repositioned/removed.
Is there any metal object underneath the motherboard when you mount it into the case that shouldn't be there? Worth double checking standoff positions and if any screws, washers etc., are stuck where they shouldn't be.
Is the board positioned correctly against the backplate? If this is not correct it can lead to a short circuit stopping the board from working.