The Greasers are a street gang appearing in Grand Theft Auto: London 1961. The Protagonist is tasked with starting a war between the Greasers & Rockers by Harold Cartwright during the opening mission of Grand Theft Auto: London 1961. The Greasers also appear as antagonists during the later missions.
The Greasers possibly make an appearance in Grand Theft Auto: London 1969 as the unnamed gang that the protagonist wipes out in Chelsea as they use the same character sprites.
They are likely based on the actual Greaser subculture, which, though North American in origin, began to appear in the United Kingdom by the early 1960's.