Etymologically, the name is usually said to derive from the oblique case of a Proto-West Germanic root: *gīsl “hostage, pledge” (see Wiktionary:Ghislain). As such, it is cognate with modern German Geisel "hostage".
The word's origin can be traced back to a Proto-West Germanic root meaning "hostage, pledge," and it shares a connection with the modern German word "Geisel", which also signifies "hostage."