From Russian ГРУ (GRU), through phonetic transliteration of Гла́вное Разве́дывательное Управле́ние (Glávnoje Razvédyvatelʹnoje Upravlénije) and acronymizing.
Meanings
The Main Intelligence Directorate — the Soviet and then the Russian military intelligence agency.
A team of investigators claim to have unmasked a deep-cover spy from Russia’s military intelligence agency, the GRU, who spent a decade posing as a Latin American jewellery designer and partied with Nato staff based in Naples. 2022 August 26, Shaun Walker, “Socialite who charmed Nato staff in Naples was Russian spy, say investigators”, in The Guardian:
Apparently nobody ever expects the GRU, Russian Military Intelligence. 2020, Ben Aaronovitch, False Value, Gollancz, page 115: