GPP should provide more long, GNU-style versions for more options. A user suggests by e-mail:

Follow also GNU conventions for program arguments.

So in addition of the nine argument -U you would accept some long arguments (more readable for occasional GPP users), e.g.

--user-macro-start=XX

--user-macro-empty-end=YY

--user-arg-start=AS
--user-arg-sep=SEP
--user-arg-end=AE

etc...

Likewise for -M.