Montevideo is one of the easiest cities in Latin America to cruise: Uruguay has had anti-discrimination law since 2004 and marriage equality since 2013, police take no interest in gay men, and the general temperature is relaxed. The scene is small but real.
Outdoors, the anchor is Parque de los Aliados in the Parque Batlle neighborhood - dense tree cover, active after dark, and the one spot every local names first. Daytime is gentler: the stretch of Rambla in front of Parque Rodo, around Playa Ramirez and Playa de los Ingleses, is the city traditional gay beach hangout - more social scene than cruising ground, and lovely at sunset.
Indoors, Dehiok in Ciudad Vieja is the city dedicated cruising bar - unmarked black door, lockers, cabins and very late hours on weekends. A couple of old cines condicionados around Centro still run their dark rows too.
The honest safety note: the risk here is ordinary urban crime, not homophobia or police. Pickpockets and phone-snatchers work dark parks and Ciudad Vieja late at night, so take nothing valuable cruising and keep your phone put away. Public sex remains technically an offence, so the usual discretion applies everywhere outdoors.