Gilbert the Bad is a character from the Knight's Kingdom line released in 2000. Gilbert is a member of Cedric The Bull's "Bull Knights". Gilbert is Cedric's expert on building attack machines.
According to LEGO Adventures! Magazine issue 15, Gilbert created the idea of using catapults to launch Cedric into King Leo's castle.
Gilbert as seen in LEGO Adventures Magazine! issue 15
Gilbert as seen in building instructions
From the Knights' Kingdom Minifigure Handbook