I'm not that fluent in python, butlayout_final.addWidget(self.widgetGL) layout_final.addWidget(self.widget)
Horizontal layouts add stuff left to right, so, if anywhere, the GL widget would be on the left, not on the right.
I don't think a GL widget has any minimum size. It might be that the other widgets have "greedy" horizontal size policies (I think a label does by default). Try setting a minimum width or an expanding horizontal size policy on the GL widget.