The UFC officially announced the bout on Monday. It will be the third in the UFC's women's division as the promotion ramps up the offerings in its newest class.
McMann won silver in the 63kg category at the 2004 Games.
After a long and decorated international career, the 32-year-old McMann transitioned to mixed martial arts. Since debuting in May 2011, she's gone 6-0 with three finishes. In her most recent bout, she defeated Shayna Baszler at Invicta FC 2.
Due to her grappling pedigree, McMann is believed by some to offer any interesting style matchup for current divisional champion Ronda Rousey, who is herself a former Olympic medalist, though she captured her prize, a bronze, in judo.
McMann's first opponent is the German-born Gaff, 23, who is 10-4-1 in her career. She will enter the fight on a three-bout win streak, with all of those victories coming via first-round KO or TKO.
St. Preux (12-5) was considered a top prospect on the Strikeforce roster after winning his first five fights in the organization. In his first major test, he lost to former champion Gegard Mousasi, though he rebounded with a third-round knockout of T.J. Cook. The fight against Villante will be his octagon debut.
Villante will ride a three-fight win streak into the bout. He's 10-3 overall. The event will be something of a homecoming for him, as he's from Long Island, New York, about a one-hour drive from host location The Prudential Center in Newark, New Jersey.