gnetlist has a command-line interface, and can be run using the gnetlist command in a terminal window. For example, a normal command to run gnetlist on a design in order to generate a list of components used might look like this:

gnetlist -g bom2 -o stepper.bom power-supply.sch motor-drive.sch

To run gnetlist, you will usually need to specify a list of schematic files to process; choose a netlist backend to use to generate the output; and provide the name of the output file in which to save the netlist.

gnetlist accepts a variety of optional arguments that can be used to alter its behaviour. They are also explained in the gnetlist manpage (which you can view by running man gnetlist).