That is a conversion of the distributed CHARMM files into GROMACS format, so presumably the change of units and computing sigma from Rmin/2 has already been performed. The units should conform to those used by GROMACS.

Sigma and Rmin/2 can be readily interconverted, but early CHARMM developers chose Rmin/2 to facilitate computing the atom pair values from the simple sum of the self terms (arithmetic combining rule).
Rick Venable
computational chemist