1. A work around I successfully used in debian, ubuntu and rhel7 is creating a short python script that's started automatically when you log into the gnome session. Note that in rhel7 the key combination to lock is super + l.
2.  
3. Create /etc/xdg/autostart/gscreenlock.py like this:
4.  
5. #!/usr/bin/python
6.  
7. import dbus
8. import dbus.service
9. import dbus.glib
10. import gobject
11. import os
12.  
13. class ScreenDbusObj(dbus.service.Object):
14.     def __init__(self):
15.         session_bus = dbus.SessionBus()
16.         bus_name=dbus.service.BusName("org.gnome.ScreenSaver",bus=session_bus)
17.         dbus.service.Object.__init__(self,bus_name, '/org/gnome/ScreenSaver')
18.  
19.     @dbus.service.method("org.gnome.ScreenSaver")
20.     def Lock(self):
21.         os.system( "xscreensaver-command -lock" )
22.  
23.  
24. if __name__ == '__main__':
25.     object=ScreenDbusObj()
26.     gobject.MainLoop().run()
27. Then make it executable:
28.  
29. chmod a+rx /etc/xdg/autostart/gscreenlock.py
30. And edit /etc/xdg/autostart/gscreenlock.desktop like this:
31.  
32. [Desktop Entry]
33. Type=Application
34. Encoding=UTF-8
35. Name=gscreenlock
36. TryExec=/etc/xdg/autostart/gscreenlock.py
37. Exec=/etc/xdg/autostart/gscreenlock.py
38. NoDisplay=true
39. NotShowIn=XFCE;KDE;
40. Comment=Allows screen locking in gnome