"GRU" is the English version of the Russian acronym ГРУ, which means Main Intelligence Directorate.
The GRU is Russia's largest foreign intelligence agency. In 1997 it had six times as many agents in foreign countries as the SVR (The SVR is the successor to the KGB's foreign operations directorate). The GRU commanded 25,000 Spetsnaz troops in 1997.
Thank Kabar.