They were on trial along with dozens of other close associates of the deposed leader, accused of suppressing protests during the uprising.
Saif al-Islam was not present in court and gave evidence via video link.
He is being held by a former rebel group from the town of Zintan that refuses to release him.
Former head of intelligence for the Gaddafi regime, Abdallah al-Senousi, is among those also facing the death penalty, as is former PM Baghdadi al-Mahmoudi.
They will be given the right to appeal, according to the BBC’s John Simpson, who is in Tripoli.
Other defendants have received sentences ranging from five years to life imprisonment.