I did a fair amount of research into this for a large religious complex. The primary manufacturer of the barcode product claims that the resolution of their codes is so tight that it can not be reproduced with standard equipment (which I find a bit suspect, but I couldn't verify).
We also looked at Transcore/Nedap equipment, but the price of the credentials x the thousands of people who would need them made it not feasible
The final plan was to use Genetec's Auto Vue, with the SharpX product triggering the gate opener with the integrated relay output. The equipment is expensive, yes, but since all of the vehicle LP information was stored digitally and there was no equipment to put on the car itself, the tail end cost would quickly work itself out
Though, keep in mind that even with a 90% accuracy (which is what I think they claim) you'll still have that 10% of people with 'strange' plates that won't be able to get in. My situation would have worked because there was a physical guard there to over ride the gate, but if your gate is unmanned, a physical credential might be the most reliable.
The other, extremely low tech way of doing it is with a pin pad. However, that's both the most convenient and the least secure. If you were going to go that route, I'd strongly suggest having a camera on the gate. We did an Open Options project like this, with HID 921 keypads at a storage facility. The cost was pretty darn low and the back end database management piece was pretty easy.
