console.log(document.getElementsByClassName('gtableheader'));

-> 0         tr.gtableheader
   length    1

console.log(document.getElementsByClassName('gtableheader').length);

document.getElementsByClassName('gtableheader')[0].style.color="red";

var x = document.getElementsByClassName('gtableheader');
for (var i = 0; i < x.length; i++) {
    x[i].style.color = "red";
}

window.addEventListener("load", function(event) {
    console.log(document.getElementsByClassName('gtableheader').length);
});