Hubert Kiechle asked the following:

has anybody ever customized Emacs to write gap code? Is there anything
like gap.el(c)? I know that I could modify a c++.el or pascal.el
to do the job, but I'm hoping, it's already out there.

In addition to Steve's answer I'd like to mention that the newest
version of Michael's GAP mode for emacs can be obtained via anonymous
ftp from
ftp.math.rwth-aachen.de:/pub/incoming/

Get the files
emacs-gap-mode-1.96.tar.gz
emacs-gap-mode.readme.

With kind regards, Werner Nickel.