I contacted Gigabyte regarding it, and this was their response:
Both IRST and AHCI should work, however, IRST is more new, while AHCI may be more stable.
We would suggest sticking to default settings if it works for you, since if the options were reset in the future due to any sort of reasons, you will be safe from having to enter BIOS again to adjust the settings.
Best Regards,
GIGABYTE