The word "gagster" can have several synonyms depending on its context. If you mean someone who makes people laugh, the proper synonym would be a comedian or a comic. However, if you're referring to someone who holds someone by force to cause harm or steal something, the right synonym would be a kidnapper, robber, or thief. On the other hand, a "gagster" could also mean a person who plays practical jokes or pranks, and the closest synonyms would likely be a jester or a trickster. Depending on the situation and connotation of the word "gagster," one must choose the appropriate synonym.