Why do you want the second GUI name in a popup menu if you want to active it immediately upon pushing the button? Anyway, you simple set the popup index in the callbacks. Let's say the string property for the popup was set in GUIDE with the strings 'CarGUI.m' and 'BoatGUI.m'. So then in the car button callback
% Set popup to index 1
set(handles.popup, 'Value', 1);
% Now launch the CarGUI function
CarGUI
And in the callback for the boat, you'd do
% Set popup to index 2
set(handles.popup, 'Value', 2);
% Now launch the BoatGUI function
BoatGUI
