If you don't have any posts in yet it's a good idea to install them first before measuring up for a gate because you may not know exactly how far apart they will be. There maybe some really hard concrete, rock, very wet dirt, an old drain or some other thing making installing a post not practical in the exact spot you want.

Measure as per the diagram below. Always check the slope across the driveway (camber) it may not look like there is any but there usually is, so it pays to check. A 360o Laser level is very handy for getting a level plane to measure from although these are a few $100 or more to buy for one that will work well outside, otherwise an old school method that works well is some clear flexible tubing with water in it attached to a reference point on one post and the other end is moved about to where ever you want to measure with the water in the tubing being visible and giving you a consistent level.

Its also a good idea to make sure the posts are plumb (straight up and down), a spirit level is good for that.  If the posts are out of plumb make notes of how much out they are so they can be factored into the design.

If the driveway drops more than 30mm consider making the bottom of the gate raked so it follows the slope of the driveway. If the top of the gate posts are at different heights you should take the lowest post and use that or if there is fencing leading up to the gate that is also on a slope then consider having both the top and bottom rails raked.

You should also check if there is any slope up from the road (rise) as this needs to be considered with swing gates.  If the driveway has a slope across it (camber) too then you will need to repeat this from both posts and at 1m grid lines across the driveway to allow for any high spots in the driveway that may not be ovious to the naked eye.  Driveways generally have the middle a little higher then the edge so water runs off rather than pools in the middle. 

... more about Swing Gates on Sloping Ground

Mounting of a Motor 

If fitting a worm drive or hydraulic gate motor you will need to position the Post Mounting Bracket for the motor accurately to get the best performance from the motor.  To do this the position of the centre of the gate rotation relative to the post is required so the dimensions are needed from the centre of the gate hinge to the inside of the post, the centre of the gate hinge to the back of the posts, the size of the posts, the thickness of the gate and centre of the hinge to the back face of the gate. Pictures always help too.