Regional jets that use the parking lot are D3, and I believe only D3, as it's set up specifically for handling multiple flights and is the most efficient way to do the buses (I hesitate to call a procedure that can regularly be referred to as a cluster f*** efficient)
I routinely flew CRJs in/out of LGA last year and some this year and more often (>75%) was using a regular gate. Parking lot on incoming is supposedly used part of the time for RJs that are early so that you can get parked and deplaned faster than waiting on your original gate. I've gotten from stopped to in a taxi in ~15 minutes with the RJ when they let you walk from the gate to the terminal. fwiw