It’s worth looking through http://manual.gromacs.org/documentation/current/user-guide/cmdline.html#commands-by-name to see what analysis GROMACS can perform.
For what you are looking to perform then http://manual.gromacs.org/documentation/current/onlinehelp/gmx-distance.html or http://manual.gromacs.org/documentation/current/onlinehelp/gmx-pairdist.html might be what you are after.