From Rap Dictionary
Gangster Crips "hit up" disrespecting the Neighbor Hood Crips, who are often referred to as "N"appy "H"eads by rivals. The letter "K" at the end stands for "Killer" and the "187" is in reference to Section 187 (often referenced in slang simply as 1-8-7) of the California Penal Code which defines the crime of murder.
The Gangster Crips (or “Tray (3x) card") is an alliance referring to various street gangs that are affiliated with the Eight Tray Gangster Crips. The majority of the sets affiliated with the "Trey (3x)" "card" derive from the original East Side Crips gang. When the war between the Rollin 60s Neighborhood Crips and the Eight Tray Gangster Crips began in 1979, surrounding neighborhoods east and west of the 110 Freeway in Los Angeles found themselves choosing sides and allegiances with either one of the two super gangs. Those neighborhoods friendly towards the Rollin 60s Neighborhood Crips (many of whom were west of the 110 Freeway) began to refer to themselves as "NHC" or "Neighbor Hood Crips" while those neighborhoods friendly towards the 83 Gangsters on the eastern side of the 110 Freeway adopted the "Gangster Crips" (or "Tray") card. This feud accounts for the majority of Crip on Crip bloodshed throughout L.A. Region. It should be noted however, that although sworn enemies, a number of Tray (3x) cliques are allied with gangs claiming the Neighbor Hood Crips (or Deuces (2x)) card. For example, the Menlo Crips were originally a part of the Gangster Crips (Menlo Gangster Crips) movement. However, they turned Neighbor Hood Crips after feuding with the Eight Tray Gangster Crips and several of their allies. Another example are the 42 Gangster Crips, 43 Gangster Crips and the 48 Gangster Crips and their strong alliances with various Neighbor Hood Crip sets such as the East Coast Crips (dubbed the "Movin Coast 2 Coast" alliance). This alliance is forbidden, and is considered an act of treason. There are many different sets claiming the "Tray (3x)" card, each with its own alliances, rivalries and history.
Also See: Neighbor Hood Crips
Gangster Crips (3x) Sets
Avalon Gangster Crips (40, 116, 88, and 53)
Broadway Gangster Crips (112, and 52)