In Act II, Scene 7 of The Merchant of Venice, the Prince of Morocco has just taken his chances and evaluated the messages on the three caskets (chests) and made his choice, choosing the gold one as being the most fitting one to contain so valuable a thing as a likeness of the beautiful Portia. He has chosen badly, drawing a "carrion of Death, within whose empty eye / There is a written scroll!" As is foreshadowed by Portia's remark in Act I, Scene 2, the gold casket holds a skeleton head and a scroll stuffed in its empty eye.
When Morocco realizes he has defeated himself by equating Portia's value with monetary gold value, he quickly takes his leave in dejection. After his departure, Portia again lets her true judgmental spirit out and says: "A gentle riddance.. . / Let all of his complexion choose me so." Complexion in this usage means skin color as the Prince of Morocco is a dark skinned Moor.
The word "gentle" is used as a descriptor of men who are suitable suitors, as in gentleman. For Portia to say "a gentle riddance," means she takes what should be a compliment and couples it with a word meaning to throw away, thus producing and ending in a veiled insult of scorn.