The clicked signal of QGroupBox has an optional checked argument that is used when the group box is setCheckable(True) to pass the state of the check box. The attached example can be used to see that PyQt passes the correct value of checked but PySide does not pass the checked argument.
The bug is easily worked around by adding the line
checked = self.gb.isChecked()
at the top of your slot (assuming you know the identity of the group box or else use a sender call).