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).