The 57-year-old actress and 29-year-old singer confirmed their involvement in separate interviews this week following American Horror Story: Hotel's finale in January.
"I do not have a clue," Bassett said of her new character on Entertainment Tonight. "I think I'm an alcoholic. That's all I know but I don't know where, why [or] what made me so. But I'm gonna be there."
Bassett played Marie Laveau on Coven (Season 3), Desiree Dupree on Freak Show (Season 4) and Ramona Royale on Hotel (Season 5). Lady Gaga made her American Horror Story debut as Elizabeth Johnson, aka The Countess, on Hotel.
"Yes, [I will return]. I can't promise you how or when or anything like that," Lady Gaga told Elvis Duran and the Morning Show on Friday. Series creator Ryan Murphy revealed he asked the star to return in a tweet in October.
"Lady Gaga is so brilliant in AHS Season 5 that yesterday I officially asked her to join Season 6. Say yes @ladygaga!" Murphy wrote.
Murphy previously told Entertainment Weekly Season 6 will be "more rogue and dark" than Hotel, which took place in 2015 in a sinister 1920s hotel. FX president John Landgraf later said the season will be "set in the present" but include "echoes of the past."
Bassett will also appear in new BBC miniseries Close to the Enemy. The actress, who shared a sex scene with Lady Gaga on Hotel, described husband Courtney B. Vance's reaction to the scene on Tuesday's episode of Conan.