The Lake County jury took just 4 hours to convict Yang of murdering Rhoni Reuter and also killing her unborn child in 2007.
Gayle, who had admitted having a sexual affair with Yang, said outside the courtroom he had his own regrets about the death of Reuter and his baby.
"At least I feel Rhoni and the baby are at peace," he said.
Gayle played for the Bears for 10 seasons.
The Chicago Tribune said a tape recording of Yang discussing the killing proved to be powerful evidence for the prosecution.
Assistant State's Attorney Ari Fisz said Yang spent months planning the death of Rhoni Reuter, who was gunned down in 2007 as she left her condo north of Chicago to go to work. Fisz played tapes of conversations after the killing in which Yang, not realizing she was being recorded, described the killing to a friend.
The friend, Christi Paschen, testified that Yang had told her about the shooting shortly after it occurred and said she helped Yang dispose of evidence, the Tribune said.