Girl cheese is not to be mistaken for GRILLED cheese, a delicious treat made by placing a slice of cheese between two slices of bread and grilling.
That being said, Girl Cheese refers to the build-up of vaginal secretions in a girl's panties; this occurs if a pair of panties has been worn for more than two days in a row. After three, the stench becomes noticeable. After 5, the odor becomes unbearable and the lining of the underwear is white and stiff.
Girl Cheese can also be used as a nickname for pretty much any STD a female can have on/in her genitalia.
Dude, she may be hot but she's a whore. I heard she has a major case of Girl Cheese.