Maybe an example is worth a thousands words:
Define this function in file foo.gmic:
foo :
echo "First comment\n"
echo[^-1] "Second comment\n"
echo[] "Third comment\n"
echo_stdout "Fourth comment\n"
Then try this from the console:
$ gmic foo.gmic foo
[gmic]-0./ Start G'MIC interpreter.
[gmic]-0./ Input custom command file 'foo.gmic' (1 new, total: 3135).
[gmic]-0./foo/ First comment
[gmic]-0./ Second comment
Third comment
Fourth comment
[gmic]-0./ End G'MIC interpreter.
Another try, with redirection of stdout to a file:
$ gmic foo.gmic foo >stdout.txt
[gmic]-0./ Start G'MIC interpreter.
[gmic]-0./ Input custom command file 'foo.gmic' (1 new, total: 3135).
[gmic]-0./foo/ First comment
[gmic]-0./ Second comment
Third comment
[gmic]-0./ End G'MIC interpreter.
$ cat stdout.txt
Fourth comment