PLUMED can be incorporated into gromacs using the standard patching procedure. Patching must be done in the gromacs root directory before the cmake command is invoked.
To enable PLUMED in a gromacs simulation one should use mdrun with an extra -plumed flag. The flag can be used to specify the name of the PLUMED input file, e.g.:
mdrun -plumed plumed.dat
For more information on gromacs you should visit http://www.gromacs.org