I see a couple problems but you may just not have posted the entire script
Rect isn’t a declared variable so the label doesn’t have coordinates and size.
You string "Thirst: + Thirst + " should be "Thirst: " + Thirst
So the whole line should be
GUI.Label( Rect, "Thirst: " + Thirst);
The error is referring the "Thirst: + Thirst + " not the Rect but it will probably be the next error you get.