Toggle between "discrete GPU only" and "automatic switching" on a 15" Retina MacBook Pro.

gpu.applescript

tell application "System Preferences"
	reveal pane "com.apple.preference.energysaver"
end tell
tell application "System Events" to tell process "System Preferences" to tell window "Energy Saver"
	set status to value of checkbox "Automatic graphics switching" of group 1
	click checkbox "Automatic graphics switching" of group 1
end tell
quit application "System Preferences"
if status is 0 then
	return "Automatic switch activated"
else
	return "Discrete graphics only"
end if