Is the fast of one upon whom a ghusl is obligatory—due to masturbation or the like—at night, then he intended to fast and fasted until sunset before performing ghusl, valid, and is Qada' obligatory upon him?

Masturbation is forbidden, and one must repent from it. The fast is valid, but you should have performed ghusl (ritual purification bath) to pray. Ghusl, making up the missed prayers, and repentance are obligatory for delaying prayers. However, impurity does not affect the validity of the fast.