gsw is an R package that provides a connection to the thermodynamic equation of seawater as defined at teos-10.org. An earlier version of gsw was referenced to TEOS-10 version 3.03, but the present one is referenced to version 3.05-4, which is current as of Aug 7, 2017. See the CRAN website for check statistics, etc.
All the gsw functions reproduce teos-10.org test values to a tolerance of 1.5e-8, the default for numerical comparison in R working on a 64-bit machine. (The tests are part of the package-building process, as is usual in R.)
Function names, as well as most argument names, match those used in the TEOS-10 documentation.