gnome-screensaver emits some signals on dbus when something happens.
Here the documentation (with some examples).
You could write a scripts that runs:
dbus-monitor --session "type='signal',interface='org.gnome.ScreenSaver'"
and that does what you need anytime dbus-monitor prints a line about the screen locked/unlocked.
Here a bash command to do what you need:
dbus-monitor --session "type='signal',interface='org.gnome.ScreenSaver'" | ( while true; do read X; if echo $X | grep "boolean true" &> /dev/null; then SCREEN_LOCKED; elif echo $X | grep "boolean false" &> /dev/null; then SCREEN_UNLOCKED; fi done )
Just replace SCREEN_LOCKED and SCREEN_UNLOCKED with what you need.