You can find the code, manual, release notes, installation instructions and test suite at the links below.

Code: ftp://ftp.gromacs.org/pub/gromacs/gromacs-2021.tar.gz.

Documentation: http://manual.gromacs.org/2021/index.html. (includes install guide, user guide, reference manual, and release notes)

Test Suite: ftp://ftp.gromacs.org/regressiontests/regressiontests-2021.tar.gz.