Lang asked: “I’m assuming that a gandhabba can stay in the gandhabba form for thousands of year straight, without birth (jati) as a human with a physical body.”
– Yes. It is possible that a suitable human womb may not be available for thousands of years!
Lang asked: “One ghost they saw was of a girl who was near her own portrait in a palace/museum they visited.”
Is this an example of attachment (tanha)? i.e. attachment to the previous physical human existence?”
– Yes. The mental body or the gandhabba, in some cases, can stay close to the last birth with a human body. Yes. That happens because of the attachment to that existence with that physical body.
– This is why there are reports of people saying that they saw the “ghost of their deceased relative.”