gap> mat := [[0,-2,0,1],[2,0,3,0],[0,-3,0,1],[-1,0,-1,0]]*Z(19)^0;
[ [ 0*Z(19), Z(19)^10, 0*Z(19), Z(19)^0 ],
[ Z(19), 0*Z(19), Z(19)^13, 0*Z(19) ],
[ 0*Z(19), Z(19)^4, 0*Z(19), Z(19)^0 ],
[ Z(19)^9, 0*Z(19), Z(19)^9, 0*Z(19) ] ]
gap> phi := PolarityOfProjectiveSpace(mat,GF(19));
<polarity of PG(3, GF(19)), >
gap> form := SesquilinearForm(phi);
< non-degenerate bilinear form >