Slope Across a Driveway

If your driveway has slope across it then the bottom rail of the gate should be angled (raked) to follow the contour of the driveway.

Raking the top rail depends on the fence line, if level then so should the top rail be, if stepped the top rail should match the lowest side of the fence line, if the fence is following the contour of the land then so should the top rail of the gate otherwise it won't look right.

One problem with Double Swing Gates with a Raked Bottom is unless your driveway slopes down from the road by the same amount or more then the gate will strike the driveway.  One solution to this is to use a single swing gate or a Bi-folding swing gate hung from the larger edge, this solution even works if the gate slopes up from the road by the same or less than the amount it slopes across.  

Slope up from the road 

If your driveway doesn't appear to have any slope on it there may be a slight slope up your driveway from the road that could prevent the gate from opening once installed. 

To check how much slope there is you will need a laser level (a cheap one from your local hardware store should be fine) so you can measure between the gate posts and up the driveway from the road. If you take measurements from opposite directions and they are slightly different the laser level probably isn't all that accurate so take some where in between that's normally accurate enough.  

If your driveway does slope up from the road but only by a small amount you can just make the bottom of the gate higher so it will clear the driveway although this gap may be too much and if its a double gate installing a hard stop may be difficult.  Alternatives are making your gate open outwards although you generally can't have it opening past the front of your boundary line or you can also have the gate open up the hill using rising hinges.

... more about Outward Opening Gates... more about Rising Hinges