Is the ghusl valid if a person suffering from waswasah (obsessive doubts) doubts his intention after performing ghusl and ignores his intrusive thoughts, and is he required to repeat the ghusl or make up for previous acts of worship?

There is no cure for obsessive thoughts, no matter how numerous their forms, except to turn away from them and pay them no heed. The ghusl (ritual bath) mentioned is valid and does not need to be repeated. Continue to strive against these obsessive thoughts whenever they present themselves to you. And every time Satan casts something of them into your heart, ignore it and pay it no attention.