This is a physical cache which you can find after completing the Geocaching Adventure "Roundabout Swindon". The cache is not at the posted coordinates. The Adventure takes in 5 interesting locations around central Swindon and Old Town.
You can find further detail about the Adventure here. The whole Adventure and this bonus cache should take you no more than an hour to complete.
The coordinates for this physical cache will be revealed after you have completed all five stages of the Adventure. You will need to be stealthy at GZ as the cache is placed in a location which is popular with Swindonians out for a stroll. Access to GZ is only available during the day - at least 9am to 6.30pm, longer in Summer.