In addition to being a somewhat famous singer, it is also a new slang term (code) for instructing someone to pick up a "gar" (dutchmaster, philly, swisher, etc.). It originated in Miami and is slowly gaining steam across the country.
Greta Garbo = "Get a 'gar, bro!"
Person A: Hey, you around? Let's burn!
Person B: Sure. Greta Garbo.
Person C: Heard! I'll stop by the gas station.