$ZPROM[PT] contains a string value specifying the current Direct Mode prompt. By default, GTM> is the Direct Mode prompt. M routines can modify $ZPROMPT by means of a SET command. $ZPROMPT cannot exceed 16 characters. If an attempt is made to assign $ZPROMPT to a longer string, only the first 16 characters will be taken.
Example:
GTM>SET $ZPROMPT="NEWZPROMPT>" NEWZPROMPT>
This example changes the GT.M prompt to be NEWZPROMPT