It's no use having yoru script start the screensaver. You need the gnome-terminal to start the screensaver. There is probably a way to pass arguments to gnome-terminal to tell it to open full-screen, and then you can use --command=myscreen-saver. But failing that, this may work:
Code:
gnome-terminal --command="wmctrl -r ':ACTIVE:' -b toggle,fullscreen ; my-screen-saver"