Gforth doesn't support multiline editing (see the manual).
A workaround is to edit a file in your favorite editor in another window and reload this file in Gforth console as:
An external file can be also edited in Gforth console via a command like:
"vim /tmp/scratch.fs" system
So a one-liner for that is:
"vim /tmp/scratch.fs" system "/tmp/scratch.fs" included
That can be wrapped into a definition as:
: scratch "vim /tmp/scratch.fs" system "/tmp/scratch.fs" included ;
So the word
scratch will open an editor and than load the edited file.
NB: if you use a quite old build of Gforth, you have to use
s" ccc" instead of
"ccc" for string literals.
[defined] _clear [if] _clear [then] marker _clear \ some definitions \ ...
Take into account that usual control-flow words can be used in definitions only.