gap> mat := [[1,0,0],[0,0,2],[0,2,0]]*Z(5)^0;
[ [ Z(5)^0, 0*Z(5), 0*Z(5) ], [ 0*Z(5), 0*Z(5), Z(5) ],
[ 0*Z(5), Z(5), 0*Z(5) ] ]
gap> phi := PolarityOfProjectiveSpace(mat,GF(25));
<polarity of PG(2, GF(5^2)), >
gap> BaseField(phi);
GF(5^2)