The GIGN (Groupe d'Intervention de la Gendarmerie Nationale) and DGSE (Direction Générale de la Sécurité Extérieure) are the French security services featured in GTA Vice City. In the world of myth hunting, these units have been related to many mysteries. The secret agents are often confused for the SWAT due to similar looking uniforms. Some members of the unit are corrupt and may leak inside information or even steal prototypes from the agencies. Such was the case of Pierre La Ponce, a former GIGN agent who betrayed the unit to steal chips for Colonel Cortez. Rumor has it that this is just one of the many cases involving the leakage of information about the agencies. A strange minor myth relating to these groups is that they are internally working with the SWAT in tracking down secret crimes around the city, and furthermore, there may be French agents living in secrecy in Vice City.
Keeping in view the kind of ammunition and vehicles DGSE is supplied with during the All Hands On Deck, it's more likely that they are supported by the SWAT. If it wasn't the case, there should have been a possible altercation between SWAT and DGSE, as the latter is operating openly on a foreign land. During the aforementioned mission, a particular glitch allows these agents to camouflage. It may be possible that they are associated with the Vice Squad.
Both units are involved in the Enforcement Nexus.