Al-Ghoul was in Gaza to attend the funeral of his mother-in law.
He writes columns for local newspapers and has frequently criticised Hamas.
Also on Friday, two people died in an explosion at a funeral procession in Gaza City, hospital officials told the Associated Press.
Thirty people were also injured in the blast during the funeral of a Fatah member, who died in an Israeli air raid on Thursday.
The source of the blast was unknown but Hamas security officials said someone at the funeral threw a pipebomb at the edges of the procession, which then bounced back into the crowd.
Gunfire and the setting off of fireworks are common forms of commemoration at funerals in Gaza.