Cities

Comparison map of Liberty City in GTA III (upper right), Vice City (lower right), and San Andreas (left), all approximately to scale.

Cities, in the Grand Theft Auto games are large population centres. Many GTA games contain only one city, but GTA San Andreas, GTA IV and GTA V contain multiple.

GTA games are replete with fictional cities, many of which are based on real-life locations:

Liberty City is based on New York City in the 2D Universe, 3D Universe and HD Universe. Liberty City is the most used city so far, but each are slightly rendered in each game and Era.

Alderney is technically a separate state to Liberty City in the HD Universe, containing Alderney City (Jersey City and Newark).

London is a rendition of the real city in (GTA London 1969 and GTA London 1961).
Manchester is a rendition of the real city in (GTA London 1969 and GTA London 1961).

Vice City is a counterpart to Miami (GTA, GTA Vice City, and GTA Vice City Stories).

San Andreas in GTA 1 is based on San Francisco. San Andreas in GTA San Andreas is an entire state comprised of three major cities: Los Santos (Los Angeles), San Fierro (San Francisco) and Las Venturas (Las Vegas). During GTA V, San Andreas is also a state, made up of Los Santos (Los Angeles) and Davis (Compton).

Carcer City is most likely Detroit or Philadelphia, and is said to be neighboring Liberty City. Whilst not featured in any game, it is mentioned in GTA III, GTA San Andreas, GTA Liberty City Stories, and GTA IV; and is the setting for Manhunt.
Cottonmouth in Manhunt 2 is likely based on Tampa, FL and has various references to other GTA Cities.
Bullworth is the setting for Bully. Has various references to the GTA Series.One for example, on television in GTA IV. Situated in New England.
Capital City is a city mentioned in the miles traveled stats of GTA IV. It is possibly based on Washington, D.C. (United States Capital) or Albany, NY (New York's Capital).

Trivia