Oxidative phosphorylation is where ATP is formed from the electron transport chain.
ATP synthase is a part of the electron transport chain but is not involved in the transportation of electrons.
The proton gradient is used by the ATP synthase that is created by the electron transport chain in the synthesis of ATP.
ATP synthase is complex V of the electron transport chain which catalyzes the synthesis of ATP from ADP and Pi by the process of oxidative phosphorylation.