GPS isn't enough. A lot depends on the receiver, antenna, how many satellites are in view, and what's going on with the atmosphere, but unaugmented GPS is generally considered accurate to about 15 meters. Not good enough for a lawn unless you're in a rural area or your neighbors don't care if you cross the property line.

GPS in your car only appears to be more accurate than that because they're typically "snapping" the GPS signal to an underlying road map.