Gabester or The Gabester is the mashup nickname used to refer to Gabriel aka The Trickster (portrayed by Richard Speight Jr.).
The name came to being after the Trickster was revealed as the archangel Gabriel in 5.08 Changing Channels.
It is frequently used by the fandom and Richard himself.
In fandom the nickname can simply refer to Gabriel or to special moments in canon or fanfiction when he is acting like his typical Trickster self.
Some fans use the term to refer to Richard himself.
Gabriel fans are often called "Gabester Girls".