Well... test on other BIOS is done, this was rather time consuming because i had wanted a full backup before test, and then been trapped by the infamous partition recovery too small from the brand new Windows 8.1...
Thanks to dmdilks, you make me re-read Gigabyte quotes on their beta bios, and eventually they state themselves beta bios as reliable.
Anyway, the results are: F13g bios doesn't allow secureboot for W8.1,
and akr1602 is right: F12 bios makes Windows 8.1 removing the warning message "SecureBoot is not correctly configured", but Windows system info confirm that secureboot is not managed at all. So: no warning message (a nice enhancement whatever), but no secureboot.
I guess i have now to figure out how to send an official request for enhancement to Gigabyte.
One more time thanks for the various answers, they were all useful. Even if currently the GA-B75M-D3H have no bios compatible with both SecureBoot and Windows 8.1 (too bad, i had planned an eDrive managed disk... on another hand this is good for my wallet ;-))