A group grading on a matrix algebra
is called
good if all the matrix units
are homogeneous
elements. We present a new way to classify good
-gradings by the orbits of a certain action of the
group
on a set of
-tuples of non-negative
integers, and we use it to count the isomorphism types
of good
-gradings on
in the case where
is a cyclic group of prime index
.