1. Brahmas cannot smell. It has only the sight and hearing. The taste, touch, and smell faculties are absent in a Brahma. That is why it is not called a “gandhabba.”
2. Yes. Even the human gandhabbas can go through walls.
3. A gandhabba can become a bit denser by inhaling orders of flowers, etc. But their “bodies” are still invisible and they can go through walls.
4. Is there a question there?
5. Since it cannot feel touch, a gandhabba cannot have sex.
- Petas are a different category belonging to the apayas, a lower realm.
- A human gandhabba belongs to the human realm. Our essence is a gandhabba; the dense physical body is just a shell. All sensing faculties are in the gandhabba (hadaya vatthu is the seat of the mind, and the five pasada rupas are effectively the five senses.
- See “Manomaya Kaya (Gandhabba) and the Physical Body“