James Gunn's Superman exists in a world full of superheroes, which includes the Justice Gang that hangs out in its under-construction headquarters, the Hall of Justice – one of the classic home bases of the comic book Justice League. But the Justice Gang heroes aren't the only ones in the hall, as there's a whole mural on the wall depicting superheroes going back 300 years (or potentially more), as established in the intro of the film – and it includes the world's first ever superhero team, the Justice Society of America.
The JSA is the real world's first superhero team. First appearing all the way back in 1940's All-Star Comics #3, the JSA was the first group of heroes assembled from across other comic books. A version of the team based on the JSA's modern incarnation appeared in the ill-fated Black Adam movie, and the characters of Kendra Saunders/Hawkgirl and Michael Holt/Mister Terrific of the Justice Gang also came to prominence in the late '90s version of the JSA.
