GL__M() was the entry point for printing a library message. There is no single replacement.
For some messages (responses that issue from template-level I6 code), you call a built-in I6 function: QUIT_THE_GAME_RM(), ANNOUNCE_SCORE_RM(), LIST_WRITER_INTERNAL_RM(), etc. The argument is a letter, but you have to look at the new template code to figure out which is needed.
For messages defined in the Standard Rules (I7 code), you invoke the “issuing the response text” activity with the appropriate response value.
Someone who has been updating this sort of extension can probably say more.