It was needed to implement rather simple operation of finding which of the placemarks in KML file particular GLatLng belongs to. To my surprise, find existing solution was tricky, so I wrote my own, see below. Code in XML parser is somewhat based on Lance Dyas GeoXML's parser.

function fetchRegions(url)
{
  GDownloadUrl(url, 
               function(doc) {
    var tree = GXml.parse(doc);
    var root = tree.documentElement;
    if (!root) { 
      alert("No regions table found"); 
      return 0; 
    }
    regions = {};
    var places = root.getElementsByTagName("Placemark");
    for (var i = 0; i < places.length; i++) {
      var name = places[i].getAttribute("id");
      var poly = places[i].getElementsByTagName("Polygon")[0];
      // XXX: we're little bit stupid here, as we only care about
      // outer boundary coordinates
      var coords = GXml.value(poly.getElementsByTagName("coordinates")[0]);
      coords += " ";
      coords=coords.replace(/\s+/g," "); 
      coords=coords.replace(/^ /,""); 
      coords=coords.replace(/, /,",");
      var path = coords.split(" ");
      var points = [];
      for (var p=0; p<path.length-1; p++) {
        var bits = path[p].split(",");
        var point = new GLatLng(parseFloat(bits[1]), parseFloat(bits[0]));
        points.push(point);
      }
      regions[name] = points;
    }
  }
               );
}

function inPoly(poly, point)
{ 
  var inside = false;
  var npoints = poly.length;

  if (npoints < 3) {
    return false;
  }
 
  var xnew,ynew;
  var  x1, y1;
  var  x2, y2;
  var xold = poly[npoints-1].lat(), yold=poly[npoints-1].lng();

  var xt = point.lat(), yt = point.lng();

  for (var i=0 ; i < npoints ; i++) {
    xnew=poly[i].lat();
    ynew=poly[i].lng();
    if (xnew > xold) {
      x1=xold; x2=xnew;
      y1=yold; y2=ynew;
    } else {
      x1=xnew; x2=xold;
      y1=ynew; y2=yold;
    }
    if ( (xnew < xt) == (xt <= xold)
         && 
         ((yt-y1)*(x2-x1) < (y2-y1)*(xt-x1)) )  {
      inside = !inside;
    }
    xold=xnew; yold=ynew;
  }

  return inside;
}

function findRegion(point)
{
  var polys = [];

  for (var n in regions) {
    if (inPoly(regions[n], point)) {
      polys.push(n);
    }
  }
  return polys;
}