I downloaded your mobo manual and it appears you are correct: There doesn't seem to be a memory voltage option. Unfortunately some motherboard model BIOS are designed this way.
Why? Who knows, but it makes absolutely no sense especially when someone like yourself really needs to see the memory voltage settings. I'm very suprised Gigabyte did this because they are a Tier 1 motherboard manufacture.
* My next suggestion is to contact their tech support directly and give them the following:
a. Mother model and Issues.
b. Minidump reads with error codes and cited probable causes, i.e. corrupted memory.
c. All diagnostic steps taken.
*** I have often found email contact, though slower, can get more indepth answers.