I'd like to run a screen if the Gnome session is locked and unlocked. Is there a way that I can intercept this and perform certain actions when the desktop is locked or unlocked?
gnome-screensaver emits some signals on dbus when something happens.
Here the documentation (with some examples).
You could write a scripts that runs:
and that does what you need anytime
Here a bash command to do what you need:
In ubuntu 14.04 the DBus event for screen lock unlock has changed and the new script for binding to screen lock and unlock events looks like the following