One difference between a gulf and a bay is quite difficult to distinguish as they both have the same characteristics. In technical terms, both a gulf and a bay are the same things, and the only difference that is pointed out is in the size. A gulf is much larger than a bay.
A gulf is said to be a large body of water that is surrounded by land and having only a narrow entrance along a strait. A bay is also a large body of water that is surrounded by land but not as enclosed as a gulf. Moreover, the bay has a wider opening than the gulf.
A gulf is a body of water in which the water has eroded very deeply into the adjoining land. On the contrary, a bay has an inward land curve.
Most of the bays are formed because of the erosion of soft rock and clay by the waves. When looking at a single gulf, it is generally fashioned along linear shores. If the shoreline is irregular and consists of complex geologic structures, a group of gulfs are formed.