The user-defined editor allows you to edit a file without having to leave Arcgsh. It is not a built-in function of Arcgsh. Instead you are free to choose a text editor with which you actually edit the file.
When you select the ``Edit file ...'' menu item from the ``File'' menu the GEM file selector box appears and asks you for the name of the file to be edited. If the editor has not yet been selected a dialog box appears to let you do so (see description of the ``Configuration'' menu).