First, get the packages gcl and libgmp3 (click the button Go To Download Page and download each file).
This will download files such as gcl_2.5.0.cvs20020625-43_i386.deb and libgmp3_4.0.1_i386.deb; then install using:
dpkg --install libgmp*.deb dpkg --install gcl*.deb cp /usr/bin/gcl /usr/bin/lisp
Now you should be able to run gcl using the command gcl or /usr/bin/gcl . You should also be able to use it from within Emacs using the command M-x run-lisp