GNU Boot seems to be Libreboot with very minor differences (mainly documentation) + removal of binary blobs.
Check https://lists.nongnu.org/archive/html/libreboot-devel/2023-0... and https://lists.nongnu.org/archive/html/libreboot-devel/2023-0... to see the patches.
There is also a Savannah repository but history does seem to have been deleted: https://git.savannah.gnu.org/cgit/gnuboot.git