Hi,
not sure about what you mean.
If you want a simple gradient pressure, use SFGRAD,... command to setup the gradient before applying the pressure. You will need to have a CSYS correctly placed for that use, it doesn't mean which axis is the reference one for the gradient (you can choose for it under the SFGRAD command).
Remember to reset the gradient afterwards, by issueing a "void" SFGRAD command (message in Output will be "Gradient has been disabled" or something like that).
If you want a law-driven pressure distribution, then you will have to learn the Function Editor and how to handle Tabular Loads.
The same applies if you want to specify discrete coord-pressure value pairs (once again, Tabular Loads).
See in the Help "How to apply Loads". It's a very long chapter, but it is very well explained.
Regards