firewns:
Kammic energy is defined at the beginning of a bhava. So, when it runs out one will leave the existing bhava regardless of which form it is in.
So, if it runs out while in the gandhabba state, the gandhabba will cease to exist and the new life-form will start.
– For example, if the new bhava is a deva bhava, then a deva will appear in the deva loka instantaneously.
If the kammic energy runs out for a human baby, the baby will die and the new life-form will start.
– For example, if the new bhava is an animal bhava, an animal gandhabba will come out of the dead body of that baby.
– By the way, this why some healthy young people just drop and die. They could have been very healthy. But of course there will be a medical reason, for example, “an unexpected” heart attack could have been the cause.