GAP Manual: 3.23. Edit

Edit( filename )

Edit starts an editor with the file whose filename is given by the string filename, and reads the file back into GAP when you exit the editor again. You should set the GAP variable EDITOR to the name of the editor that you usually use, e.g., /usr/ucb/vi. This can for example be done in your .gaprc file (see the sections on operating system dependent features in chapter Getting and Installing GAP).