var gmaps = document.getElementsByName("gmap_nl");
for (var gmap_num=0; gmap_num<gmaps.length; gmap_num++) {
document.getElementById("map_"+gmap_num).style.height = (y.size - 70) +'px';
}
warum nimmst du hier nicht
gmaps[gmap\_num].style.height = (y.size - 70) +'px';
statt getElementById zu bemühen?