System fan control Gigabyte

My mistake!.. I thought I'd read AMD somewhere in your opening post... so, you won't find the same settings I suggested in a Intel board BIOS, but they may have similar tittles. You found the Fan speed control so that's what you were asking about. If you need to do further fan speed management, and you don't find more settings in the BIOS, you can use the Widows Control Panel \ Power Settings \ Power Plans... select the power plan you have active, and Change the Advanced power settings... go to the Processor Power Management \ System Cooling Policy \ Setting: Active... to make changes to the setting: click on "Change settings that are currently unavailable" (above the settings window). The setting you choose for the Minimum and Maximum processor state, will influence the Fan speed... while the System cooling policy is "Passive" the fan speed will remain constant.

Edit:
Something else I messed is that you have a water cooler with the fan mounted on the case, so the controlled fan speed will depend on the motherboard power header you connect it to. On the CPU fan header it should act and respond to the CPU management settings, exactly the same as the stock cooler fan.