Gandhabba can certainly generate vaci sankhara.
A gandhabba, in general, can hear.
– But there is no mention in the Tipitaka about a gandhabba being able to attain a magga phala.
Thus, it could be either that a gandhabba would not listen to Dhamma.
– Another possibility: A deeper point is that since a gandhabba does not have a physical body and a brain, citta can just flow “without control”.
– The brain needs time to process sensory data, and thus slows down the whole process. That gives us (those with brains) to take a bit of time and contemplate the consequences.
See, “Brain – Interface between Mind and Body”