I would suggest Albers equal-area conic as in this example for the UK and adjust the parameters for Poland.
First generate your topojson from the GML file with the following two commands (note that the the geographic coordinates in gml are swapped, so we need to tell ogr2ogr swap them back with the --config
options):
ogr2ogr -f "GeoJSON" --config GML_INVERT_AXIS_ORDER_IF_LAT_LONG YES --config GML_CONSIDER_EPSG_AS_URN YES poland.json poland.gml
topojson -o pl.json poland.json
For Poland you want to set the standard parallels to something like 48°N and 56°N, rotate longitude by -19° and set the center to 0°W 52°N.
The complete code is:
<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="http://d3js.org/d3.v3.min.js"></script>
<script src="http://d3js.org/topojson.v1.min.js"></script>
<script>
var width = 600,
height = 600;
var projection = d3.geo.albers()
.center([0, 52])
.rotate([-19, 0])
.parallels([48, 56])
.scale(5000)
.translate([width / 2, height / 2]);
var path = d3.geo.path()
.projection(projection);
var svg = d3.select("body").append("svg")
.attr("width", width)
.attr("height", height);
d3.json("pl.json", function(error, pl) {
svg.append("path")
.datum(topojson.feature(pl, pl.objects.poland))
.attr("d", path)
.attr("fill", "#eee");
});
</script>
