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 >