The Marriott does offer a "discounted" room rate if you are participating in the show. But the parking still sucks ($24 a day avg).
Here's a trick I learned a couple years ago, thanks to a dear friend who lives in Oakland.
Park at the BART station (I can't remember which one, but there is one in Oakland that is in the nice area of town - I will find out the name) and then take the BART back to the hotel - there is a BART stop right across the street from the Marriott. The parking at the BART was MUCH cheaper than at the hotel, and my vehicle was safe there for the whole three plus days when we were at the Gathering.
When it came time to check out, one of us took the BART over to pick up the car, and came back to load everything up at the hotel. Easy peasy.
Edited to add: I THINK the BART station is the Rockridge station, but I am still trying to get in touch with my buddy in Oakland who referred me...