“In the gadhabba state, there is probably no chance to be in the bhavanga state, and no such thing as “sleep”?”
A gandhabba very likely does not sleep. There is no need to sleep since there is no physical body that needs to “reset and recover” from the physical activities during the day.
– But the mind could be in bhavanga state if there is no arammana (thought object).
Another thing to think about: A gandhabba inside a physical body is the same as a gandhabba outside.
– The only difference is that a gandhabba inside a body would not get a sensory input if the body is asleep.
– A gandhabba outside a physical body gets sensory inputs (only visual and auditory) without such a restriction.