gschem can be launched directly from your system applications menu: look for “gEDA Schematic Editor”, which can be found in the “Engineering” or “Electronics” submenus on most Linux systems. It can also be run by opening a schematic file (ending in .sch) from a file browser window.

Alternatively, you can run gschem from the command line using the gschem command. You can run it without any arguments just to open a new, untitled schematic:

gschem

or you can specify the names of some schematic or symbol files to open:

gschem mydesign.sch symbols/resistor-1.sym

If any of the schematic or symbol files you specify don't already exist, gschem will create a new, empty file with that name.

gschem accepts several optional arguments that can be specified on the command line to alter its behaviour. They are also detailed in the gschem manpage (which you can view by running man gschem).