I have no idea about how to use it in gnome-software but I assume it is like discover.
If there are updates for your firmware they will appear along with regular system updates, if not you will see nothing.
LVFS is enabled by default and I do not recommend people enable the optional testing repo listed in that screenshot, firmware is not something you want to be a guinea pig for there is a chance of bricking your hardware any time you mess with firmware.
Note that not all firmware is supported by fwupd/LVFS. For example it does not handle UEFI updates for my custom built PC with an Asrock motherboard but does for my logitech wireless keyboard dongle.
Devices supported by fwupd are listed here: https://fwupd.org/lvfs/devices/