A Gimp Suit is an article of clothing worn by Carl Johnson in Grand Theft Auto: San Andreas to attract Millie Perkins as a girlfriend. The gimp suit is black bodysuit with gloves, shoes, red bikini briefs and a gimp mask. It does not affect Respect and Sex Appeal when worn outside of any mission.
If dating Millie Perkins with this suit, CJ will get a special date and will directly be invited for coffee upon arriving at her house, quickly increasing the relationship percentage to eventually get her key card as a requirement for the Caligula's heist.
The Gimp Suit could be a reference to a similar suit worn by a silent character in the 1994 film Pulp Fiction.