Try the following:
1. Disable any unused SATA ports in BIOS. Only one that should be enabled is the one that boots OS.
2. Plug the GPU directly into the board with all the pcie cards off the rig and see if it comes up under "other devices". Once it's recognised and the driver is installed add all the other cards on either all in one go or in 2 etc..
3. Contrary to the mining guide by Gigabyte, enable gpu display to the 16x pcie and disable internal graphics. Essentially, try disabling anything that could potentially using the pcie lanes or bus lanes.
4. Try rotating between the Gen speeds. I started with auto and had to work through each with risers. None of them worked though and ended up just leaving it on Gen 2 trying step 2.

I also have a Gigabyte mobo, not your model, but I spent the same amount of time trying to get pcie slots to recognise a MSI 1060 or recognising the gpu. The only way I've gotten it to work was option 2. I talked to a tech friend who has lots of mining experience about why the board is picky in that way and he said it must have something internally about the way the mobo chipset assigns the pcie lanes. The only thing I can think of in your case is that it won't assign lanes to the 16x if the others are filled first. This explanations makes no sense to me - because I literally tried everything to get mine set up, but there must be some logic behind why it will only recognise them in a particular order/assigns them. What order is anyone guess.

In my experience, I always fill the 16x first, then fill the others. The first GPU plugged straight into the mobo - then if you're lucky swap it out for a riser and it should still be ok. If not - my only other guess is that the riser cards have some compatibility or recognition issue on the mobo.

Hope this helps - also let us know if you've fixed it I want to know how you got it to recognise because I want that board.