Seas tend to be larger than gulfs, which are often smaller oases of a sea or ocean. Seas can be landlocked while a strait will always link gulfs to a larger body of water. The main difference between a gulf and bay is their size. A gulf is much larger than a bay.
A gulf is a sizable area of water that is surrounded by land and having only a narrower entrance along a strait. A bay is also a body of water that is surrounded by land, but it has only limited access along a strait. A bay is also enclosed as a gulf.