Go here and select the Windows version:
http://www.gigabyte.com/products/product-page.aspx?pid=2693&dl=1#driver
However, it appears that Windows 8 is not supported on the GA-G31M-S2L, so the SATA driver is not your only problem.
If you did manage to get Windows 8 installed, Gigabyte have no Windows 8 drivers for that board so most of the on-board hardware won't work properly, if at all.
If the motherboard manufacturer doesn't have Windows 8 drivers for that board, you can stop looking.