Different solution, so different answer.
If you're looking for sensor-based, you can do pretty much anything. Passive sensors (RFID tags) are, in my experience, only reliable over very short distances; 1 meter or less. Additionally, RFID tags work best when the reader is located normal to the face of the tag. That is, if you glued the tag to a windshield, a reader on the side of the car is the worst place you could put it. You want the tag "broadside" to the reader to get the best signal strength (again, range is a big issue), which would mean either put the reader 1m in front of the windshield or put the tag on the driver/passenger window.
You could do RF, over something like XBEE, or you could do infrared like a TV remote. Anything that's not passive, which again is essentially only RFID, requires power. This means batteries, which could (would) be drained quickly if they are constantly transmitting, or splicing into the vehicle power wiring, which could (would) be costly and time consuming.
What are your requirements? What's your intended range? How many vehicles are you servicing? Do you have a budget? Can you splice in to vehicle wiring? Do you need automation, and if so, what degree of automation?
Your question is very open-ended.
For a sensor-based system, the easiest solution would be a garage door opener or similar. This would require a small transmitter in every vehicle, where the driver pushes a button to send a signal to open the gate. You could also just put a keypad at the gate, where the driver has to enter a passcode.
Basically, any transmitter that could run on batteries should only run intermittently, requiring interaction from the driver. Any transmitter that emits constantly will need vehicle power. I believe that you will find that any passive transmitter is simply not feasible due to the range requirement. Also, RFID readers are expensive, much more so than the garage door openers, but probably less than interfacing with vehicle wiring.
If you're insistent on a totally passive system, I would look at simply upgrading the security camera. This would provide you the higher resolution you need to do OCR. At that point, see my solution above of bumper stickers if license plate reading is not feasible.
Know that, whatever the system, there will be failures, and you will always need a gate operator on standby to handle the failures (guests, bad transmitter, battery died, dirty sticker, OCR didn't register, etc. etc.)