John Gill Commentary Luke 8:36

"And they that saw it told them how he that was possessed with demons was made whole." — Luke 8:36 (ASV)

They also which saw it
The disciples of Christ, or the men of the ship, or persons who lived hard by in the fields, who were eyewitnesses of these several things:

told them by what means he that was possessed of the devils was
healed ;
(See Gill on Mark 5:16).