The Costa Rican Gang is a gang of Costa Rican drug smugglers who are attempting to move in on Diaz's patch. Their leader is about to get the fastest boat in the city, allowing them to get to drug deals faster than Diaz's men, so Diaz sends Tommy Vercetti to retrieve the boat in the mission The Fastest Boat. They wear black leather jackets with a red under shirt with gray jeans. Their skin model can also be seen almost everywhere in Vice City. This is the only appearance of the gang in "gang form", otherwise they are seen in Vice City as pedestrians.
Possible Connection to Cuban Gang
It is hinted the Costa Rican Gang is in fact the Cuban gang with different character model. Killing members of the gang while failing the mission multiple time, changes the least favorite gang statistics to the Cuban Gang. Also if other gang members are lured, every gangs will engage in combat with the Costa Rican Gang excluding the Cuban gang. Shots fired by Costa Rican gang kills other gang members except Cuban Gang and vice versa. Another hint is both the Costa Rican Gang and Cuban gang share a similar interest in boating business. The immediate next mission by Ricardo Diaz: Supply and Demand features the Cuban Gang as the rival, once again after the very boat Tommy stole from them. The "Costa Rican gang" term is never used in the game. Thus it is possible that the Costa Rican Gang is in fact the Cuban Gang with a different pedestrian/character model.