My dad has a long history of gout. His foot and knee have been pussing. What could this be m what could cause it?

Tophi. If the discharge is white and is not associated with pain or swelling, it is possible that your father has tophi lesions which are common for long-term gout patients. These are benign. But in any case I would have a doctor look at them to confirm.
Not Gout. If it has pus draining...It isn't gout. It needs to be checked out right away. You need to make sure there isn't an infection.