Automatic driveway gates get very hard use.
They break all the time.
And they are NOT a place to try to save money.

I used to have a neighbor who made his living fixing them- he had a complete machine shop and welding shop, a service truck with a welder and compressor- Probably 100 grand's worth of equipment. This was in the mid 80's, and he still managed to charge close to $100 an hour, and he worked all day every day, sometimes 6 days a week. He was a popular guy.

These mechanisms wear themselves out, and thats without people hitting them with cars, or trying to manually wrench them open or shut.

If this is a paying job for someone else, I would strongly advise hiring a professional gate company to do the mechanisms. I have done this several times, and it is, in my opinion, the only way to go- somebody else gets the headaches of those 11PM calls on Sunday night, somebody else has to explain the parts will take two weeks to come in, somebody else has to rebuild the same gate opener for the third time.

If it is for yourself, the simplest is a chain drive sliding gate- these are relatively fool proof, although they, too break and wear out with remarkable frequency. But they are the cheapest- an ordinary electric motor, a chain drive to a gear reduction unit, and then a chain drive to the gate, with the gate rolling on an upside down angle. Of course, in some locations and setups, it must be a swinging gate, not a slider- more money, fussier install.

Hydraulic cylinder swinging gates get complicated and expensive very very fast. I did a huge one once, for a celebrity in Hollywood, that was a 20 foot gate, and the hydraulics alone, with remotes and wiring, ran over 20 grand. Lucky for me, it was not in my contract- I did the metalwork only.

The big Ornamental ironwork suppliers sell basic, lower end gate openers- like Jansen
http://www.jansensupply.com/pdf/pg170-178.pdf
or Classic Iron
http://www.classicirononline.com/gate_operators01.html

But most clients want more- more features, more power, bigger, faster, more remotes, and so on- and then you need to get into systems like DoorKing, one of the oldest and best around-
http://www.doorking.com/pages/home.php

Very quickly, it becomes a computer job, not a metalworking job.

A buddy of mine, a very talented and experienced metalworker, just did a gate like this- he figured, "how hard can it be?" and bought the openers, for a couple grand- and lost his shirt- days of fussing, installing, fine tuning, rebuilding- he figures he should have doubled his overall bid to cover the time it took him to install and troubleshoot the gate opener.