Gloucester Park is easy to get to. Can get there via either A13 or A127, although the A127 is closer.
Route would be:
M25 to J29
A127 towards Basildon/Southend
Leave A127 at Basildon exit onto A176, head towards town centre
Gloucester Park is on your left - I assume the fields will be setup in the top section of the park where they stage the Race for Life?
Less than an hour (depending on traffic) from Stansted (M11, M25, A127). Probably a better choice than London City, as the A13 out of London can be a bit of a 'mare.
Southend Airport is closer than both, but I think they only do passenger flights to/from Jersey.