Authorities have said Gaines and two others attacked Scotty Joe Weaver at his trailer in 2004.
Prosecutors said they beat, strangled and cut the 18-year-old before setting his body afire, and the extent of Weaver's injuries pointed to the attackers' distaste for his sexual orientation.
Gaines will not testify at the trial set to start May 7, but jurors will watch a video recording of his confession, defense attorney J. Clark Stankoski said. Lawyers have not said whether Gaines will testify against his two co-defendants, Robert Porter and Nichole Kelsay, who are scheduled to be tried separately later this year.
The slaying drew interest from gay rights groups, and hundreds of mourners attended a vigil for Weaver in Mobile after the killing. Anti-gay groups picketed outside the Crossroads Church of God, where Weaver's funeral was held.