Sloping grades are dealt with  in a hierarchy of solutions.
1)  A slight slope is either ignored or, when feasible, the grade is feathered to an aesthetic acceptance.

2) A more marked slope will often opt for a black rubber sweep fitted to a groove in the bottom of the gate.  This prevents the "floodwater cuffs"syndrome in which one edge of the gate appears to be floating.  The sweep folds over as the gate opens and the grade rises.  And yet the symmetrical integrity of the gate is preserved and this is what the eye registers, not the sweep.

3)  An even sharper slope, what we'll call Tier 3, is where the bottom of the gate runs parallel to the grade, while the rest of the gate is square and true to its geometry. 

4) Finally, when the slope is dramatic enough to threaten our equilibrium, we opt for the above image in Ventura, where we have a parallelogram where all horizontal lines are parallel to the slope.  At this angle, it is impossible for the most unassuming eye to ignore the drop and its relation to the geometry of the gate.  So we give in and bend to the inimitable forces of nature:  A hill.