You could well be correct in thinking this is a motherboard BIOS issue, especially as there are some problems with a few of these newer GPU's which have only be resolved with a BIOS update. With regard to the PSU, as you are most probably aware, this is the most common cause for this kind of issue and it is always worth double checking as most people don't realise that even a new PSU might not be OK.
I think, it might be best for you, if you contact Gigabyte Tech Support here: http://www.gigabyte.com/support-downloads/technical-support.aspx
as they would be in a better position to answer your query then the general users of the forum. It can take 7 - 10 days before you get a response from them, so be patient.
I hope you can get this figured out and, when you do it would be great if you could post back here with the result.