You can do the following:
1. build the gcl port
2. edit the makefile for the Maxima port and uncomment the line that
specifies using gcl as the lisp (and comment the cmucl line).
Usually, the FreeBSD ports allow you to set variables to give options
for building the port (such as the type of lisp to use), but this port
doesn't seem to allow for this so you have to manually edit the makefile.
After all is said and done, it works perfectly.