J Source is available under GPL 3.
The core J engine was not changed in J801 so the engine J701 source is the latest. J801 has significant changes in other parts of J, but not in the core engine.
Download j701_b_source.tar.gz to [temp].
$ cd ~ $ tar -xzf [temp]/j701_b_source.tar.gz $ cd jgplsrc $ cat docs/readme.txtJoin the source forum. Have fun!
J Source is also available under Commercial license. Contact us for more information if a license without GPL restrictions is of interest.