I'm guessing "Death in Venice" by Benjamin Britten, starring Peter Pears as Aschenbach (though I wouldn't describe him as openly gay).
That's a good guess, Meryl, but it is actually
The Knot Garden, composed by Michael Tippett (1905-1998) to his own libretto, and which premiered at London's Covent Garden on December 2, 1970. The character is Dov, and tenor Robert Tear created the role. As Dov's lover Mel (created by Thomas Carey) is also a character in the opera, one could argue that this opera introduced
two more-or-less openly gay male characters to the genre, but Dov is always the one cited in this context.
Tippett later organized some of Dov's arias into a separate work called
Songs for Dov.
Incidentally,
Death in Venice premiered at Snape Maltings on June 16, 1973.