Greetings!
GCL will use GNU readline by default, and this is a good thing
(e.g. command completion, command-line editing). GCL will
automatically determine if the controlling terminal is readline
capable (e.g. vt100, xterm ...) before enabling readline, and bypass
readline initialization if the terminal is too 'dumb' (e.g. emacs
shell buffer) In the latter case, Control-d works just fine -- in the
former it cannot, because readline needs to map the control keys. I
suppose a nice feature to add would be an environment variable which
could disable readline initialization, so that one could get this
behavior as desired at runtime even on xterms, etc. But really, I
think you'll get a lot more out of readline in maxima than you lose
with Ctrl-d when readline is initialized.