RORY: Well, community colleges have ceremonies.
LORELAI: My community college doesn’t even have a lawn, they won’t necessarily have a ceremony.
Rory is correct: community colleges have graduation ceremonies, like any other college. You don’t need a lawn – they can be held in an auditorium, theatre, gym or even a separate hired venue.
The fact that Lorelai’s college isn’t having one, only her business class as a separate event, is obviously just to fit in with the restrictions of filming the episode.