A term used in exclamation. Gash is a word commonly associated with vagina, and the cake is just another word used to place an element of randomness into the term. It's rather a random comment, and can be used to refer to any of the following: vagina, pussy, a cake of cunt, a hot girl.
Jose: Yo dude, you is a whole lot of gash cake.
Yuggio: You chatting breeze Jose, go get a slice of gash cake from that chick (points to a fat women wearing an ACDC tee-shirt, who has blataatly got a whole lot of gash cake - the icing alone is fucking huge).
Is a much more crass and crude way of saying 'sex life' or 'love life', but really only applies to males (and possibly lesbians), as the word 'gash' means vagina and the term is basically enquiring whether he (or she in the case of a lesbian) has been scoring with any females.
There is also an alternative interpretation of the term, meaning the 'life of gash' - whereby someone focuses on scoring gash and uses any means necessary to get gash. Someone like this may be known as a gash hound.
Jerome: How's the gash life, matey?
Abdul: Dude, it's non exsistent.
Jack: Did you get some gash last night?
Thomas: No, I need to get some better chat up lines and improve my gash life, I'm getting desperate.