LORELAI: They’re gonna get to watch you walk down the aisle wearing your cap and gown and get your diploma and go to Harvard and be a Phi Beta Bimbo and graduate with honors and just set the world on fire.
Lorelai is referring to college fraternities and sororities, also known as Greek letter societies, as they always have names consisting of Greek letters. Several begin with Phi Beta, and Lorelai adds a “Bimbo” to show what she thinks of the female students who would join such an elitist sorority.
The Annotated Gilmore Girls 