You currently have
function GLP_fsize_val_Callback(hObject, eventdata, handles)
handles.GLP_fsize_val=str2double(get(hObject,'String'));
guidata(hObject, handles);
This is replacing the stored handle to the fsize edit callback with the numeric equivalent to the string stored in the callback. That would invalidate the use of the stored handle elsewhere.
There is no need to have a GLP_fsize_val_Callback callback at all unless you want to do error checking on the string the user entered. Callbacks for edit boxes allow you to do error checking, and they allow you to do other actions (like calculations) when the user enters data. You have your Apply button so you do not want calculations done when the user enters data, so the only point in having a callback for the edit box would be error checking.
