var basex_gk = 3542000.63492063480000;
var basey_gk = 5913999.75000000;
var gkpp = 1.26984126984127;

function zeiggauss(e){
  e = e || event; 

  var tx = (e.clientX + viewx + mapx*tilewidth)/aktscale;
  var ty = (e.clientY + viewy + mapy*tileheight)/aktscale;
  var gkx = tx*gkpp+basex_gk;
  var gky = basey_gk+52000-ty*gkpp;
  prompt("Gauss-Krüger-Koordinaten",Math.round(gkx)+","+Math.round(gky));
}