Usually a gate will stop non members from coming on your property and flying. If they can't get their car in it is not likely that they will be so determined to fly that they will pack their support gear and plane in and out for a flying session.
Gates won't do much good for vandals and thieves as they have a very different agenda.

As someone else pointed out here, there are those that want to fly without paying their share for the privilege.
They seem to use some form of the theme that includes the idea that "I have the right to fly because I have a model." The fact that they can afford a model and transportation but not money for a place to fly is almost irrevelant to this type of person. They seem to think that your place to fly should be open and free to them because they just don't have the money today but it sure is good flying weather.