It is based on TK 3.6 and TCL 7.3 and available from ftp.cs.berkeley.edu and many mirrors. You probably want the highest XX version number. In the following this compressed tar file is simply referred to as gcl.tgz. To make the whole system, if you have obtained gcl.tgz. There may be occasional warnings from the C compiler, but all files should compile successfully producing .o files.
Work needs to be done.
A new compiler has been written, which is closer to the ANSI standard and provides some other benefits. It will be in a future release. We will need people willing to beta test and isolate any bugs. Make it easier to link in C code such as Novak's window stuff. Introduce C level and Lisp level way of signalling errors of the types specified by the ANSI standard.