Memy way to say gamble. The word was popularized by Twitch streamer named Reckful (Byron Daniel Bernstein), when he was playing online casino games on his stream.
A girl that has a nice body, but also had a terrible face.
It is named 'gamba' because you can eat a gamba(crustacean), but when you are eating you have to put away the head.
So you trow away the head and eat the nice part.
You can classify your relationship with a girl into two broad categories. Either she is your gamba or she is your doyin. Your gamba is a cute girl that you like and are dating, but you the relationship is usually (but not always) limited to more hedonistic pleasure. If the girl is very close to you to the extent that you confide in her your deepest insecurities and you are getting extraodinary feelings of love and affection for her, then she is you doyin.