GAP Manual: 34.3. IdentityMat

IdentityMat( n )
IdentityMat( n, F )

IdentityMat returns the identity matrix with n rows and n columns over the field F. If no field is given, IdentityMat returns the identity matrix over the field of rationals. Each call to IdentityMat returns a new matrix, so it is safe to modify the result.

    gap> IdentityMat( 3 );
    [ [ 1, 0, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ]
    gap> PrintArray( last );
    [ [  1,  0,  0 ],
      [  0,  1,  0 ],
      [  0,  0,  1 ] ]
    gap> PrintArray( IdentityMat( 3, GF(2) ) );
    [ [  Z(2)^0,  0*Z(2),  0*Z(2) ],
      [  0*Z(2),  Z(2)^0,  0*Z(2) ],
      [  0*Z(2),  0*Z(2),  Z(2)^0 ] ]