As already mentioned in the introduction of this chapter matrix rings are
after all rings. All ring functions such as Units and IsIntegralRing
are thus applicable to matrix rings. This section describes how these
functions are implemented for matrix rings. Functions not mentioned here
inherit the default group methods described in the respective sections.
IsUnit( R, m )
A matrix is a unit in a matrix ring if its rank is maximal (see RankMat).
GAP 3.4.4