You can use the g_angle tool:

g_angle -f -ov -all -n -type dihedral

You have to prepare an index file with quadruples of atoms constituting a dihedral. The following link describes which atoms to select:

http://www.msg.ucsf.edu/local/programs/garlic/commands/dihedrals.html

The first column of the output file contains the average of all dihedrals, the following columns contain individual dihedrals in the order given in the index file.