Portable petrol generators are self earthed, and as such it is not necessary to have an earthing device. This includes all the SGS brand generators. Only you start using a generator about 10.0KVA or above you should be using an earthing spike.
Some locations will require that you use an earth spike, but this is more often if the generator is to be in the same location for a prolonged period of time. If you are required to use an earthing spike, always take care as you may drive the spike into existing piping or cabling underground possibly causing accident or injury.