In the Flight Manual, if you have to do the sums yourself.
Otherwise, your company will (should ?) have the data either implicitly (buried within the landing weight data) or explicitly (as a separate set of climb data) in the Operations Manual.
As I recall, the Airbus doesn't have a paper performance data set so the information will be in the approved computer program
Those gradients are not airport related, they are Airworthiness related.
Yes .. and, no. The basic WAT gradients are out of the design standards but, if that doesn't get you over the hilltops, you need to go into the AFM data to figure out lower weights which will provide a better gradient to suit the terrain.