Rainbow Six Wiki

"The Urban Tactical Response Team, codenamed Grim Sky (or GSUTR) is Rainbow's testbed for operations in dense urban centers. It draws on such expertise as riot-officers for mob behavior and intelligence operators for converting local assets and building allied networks. The core operators are Maverick and Clash, but the unit also employs the expertise of Caveira, Bandit, Alibi, Castle, and Ying. Grim Sky trains operators on navigating and surviving in hostile cities or evading and diverting mobs."
Siege In-game description

The Urban Tactical Response Team, codenamed Grim Sky (GSUTR), is a counter-terrorism unit of Team Rainbow. It was established to operate in dense urban environments.

It is an available organization introduced in the Operation Grim Sky expansion of Tom Clancy's Rainbow Six Siege.[1]