I wonder if a gem / a sentry ward or dust reveals Slark's Ultimate "Shadow Dance".
Slark is not entirely invisible, as one can still see the little cloud he hides in. This means that he can't be directly targeted rather than being plain invisible.
As to answer the question: No, GoTS and Wards do not reveal Slark (neither does anything else).
Plenty of spells can hit him while "Invisible" though, as stated here