If the program is correctly installed then you start GAP by simply
typing gap at the prompt of your operating system followed by the
return or the newline key.
$ gap
GAP answers your request with its beautiful banner (which you can
surpress with the command line option -b) and then it shows its own
prompt gap asking you for further input.
gap>
The usual way to end a GAP session is to type quit; at the gap
prompt. Do not omit the semicolon!
gap> quit;
$
On some systems you may as well type ctl-D to yield the same effect.
In any situation GAP is ended by typing ctl-C twice within a
second.
GAP 3.4.4