When I went several times I parked one block off of Golden Gate, right at the railroad tracks, no parking fee. It is but a short walk to the gate from there.
If I recall my directions right, on the west side of the fairgrounds is a large parking lot, it was always full when I went to the swap. But there was a large open lot on the east side, but a longer walk. Pretty sure there was a parking fee too back then. Cash only.
The yellow field to the south west is General Parking FREE, then you enter just east to walk in. Easiest but you have to get there before 8 am to park, it fills up fast. A lot of people park along the RR track and walk into the north entrance (yellow dot) or east entrance (yellow dot), again, parking fills up fast in surrounding neighborhoods.