The theory of gravity, as described by Einstein's general theory of relativity, is not a special case of string theory. However, string theory does incorporate gravity as one of its fundamental forces.
String theory is a theoretical framework that attempts to reconcile quantum mechanics and general relativity, which are two fundamental theories of physics that currently describe different aspects of the universe. In string theory, the fundamental building blocks of the universe are not point-like particles, but rather tiny, vibrating strings.
One of the key insights of string theory is that it predicts the existence of additional spatial dimensions beyond the three spatial dimensions we experience in our everyday lives. These extra dimensions, typically six or seven in number depending on the specific version of string theory, are compactified or curled up at extremely small scales.
Gravity is naturally included in string theory because the vibrations of these tiny strings can give rise to particles that behave like gravitons, the hypothetical particles that mediate the gravitational force. In this way, gravity emerges from the interactions of the strings in string theory.
However, it's important to note that string theory is still a subject of active research, and many of its details and implications are not yet fully understood. While it has the potential to provide a unified framework for describing all fundamental forces, including gravity, it has not yet been confirmed by experimental evidence, and many aspects of the theory remain speculative.