I would be pretty careful if I were you about gardening on top of an old gas station. They used to use really crummy underground tanks and gas would seep into the dirt. If I were you I would take soil samples and get them examined. You might even be living on a superfund site!
You can still have a garden though. One idea is to lay down a liner, make raised beds, and get a truck to dump a load of topsoil in them.