function Button1_onclick() {

var grid_ID = “ctl00_ContentPlaceHolder1_GridView1″; //assuming that this is the generated ID
var grid_obj = document.getElementById(grid_ID);
if (grid_obj != null) {
var new_window = window.open(’print.html’); //print.html is just a dummy page with no content in it.
new_window.document.write(msg)
new_window.document.write(grid_obj.outerHTML);
//new_window.print();
//new_window.close();
}
}

The code is supposed to copy the ‘content’ of your Gridview, then pasted it in a new window. The new window is the one that will be printed.

I think somehow you copy the whole content of the original page, instead of only the ‘outerHTML’ of the Gridview.