With Gravesend being in Kent, you'll need to take the M25 south/eastbound towards the Dartford Crossing. Once you have crossed onto the other side of the Thames and passed through the tolls, exit at the first junction for the A206 (Erith and Swanscombe might just be on those signs).
Continue following the signs along the A206 for the Bluewater shopping complex until you reach the junction for the A226, which has a McDonalds located to the top left of the roundabout. Take the first exit and head for Northfleet. After 1.5 miles will be a junction with Grove Road.
Turn left here and the ground is not far away.
Tools: AA Route Planner | Highways Agency
There is a small amount of parking available behind the away terrace, at a cost of £1, but otherwise you can risk your chances on the roadside, either on the main road, or on a side street leading to an industrial estate.
Station: NORTHFLEET
Served by: SOUTH EAST TRAINS
Services to: LONDON BRIDGE
The station in Northfleet is just a walk of 500 yards from the ground, provided you exit left from the station - otherwise it could be much longer.
Tools: National Rail | tubeplanner.com