Hi Charly, why don't you target the parent div instead.
@media only screen and (max-width: 959px) and (min-width: 768px){
.gw-gopf-post {
width:50% !important; /*or pixels*/
}
also there's a missing bracket at the end (the one that would close the media query).. check if that's a copy/paste mistake or if is in the code.