If the margin values of DocumentSource is set to 0, gaps will decrease very much. However, compared with PrintTask by C#+XAML, a top margin(Probably header/footer area) is made a little.
var printTask = printEvent.request.createPrintTask("Print Sample", function (args) {
var src = MSApp.getHtmlPrintDocumentSource(document);
src.rightMargin = 0;
src.leftMargin = 0;
src.topMargin = 0;
src.bottomMargin = 0;
src.shrinkToFit = false;
args.setSource(src);
}
In my environment, when this CSS was used, it has removed the margin without the necessity for src.rightMargin, etc.
@page {
margin:0cm;
}