Welcome to Glen Campbell Forums, FireCooky. Great question.
Glen's performance of "Gentle On My Mind" (how about that outstanding guitar solo!) is from a Canadian television series "In Session".
The series was filmed live at CHCH TV Studios in Hamilton, Ontario, Canada.
There were 26 episodes in total, 13 from 1983 and 13 from 1987.
The segment you posted with Glen and Leon Russell is from one of the In Session episodes featuring these two artists in 1983.
We believe the Session tracks included:
I saw the light / Gentle on my mind / Heartbreak hotel / They still dance to waltzes in England / Rollin' in my sweet baby's arms / Jambalaya / A song for you / Southern nights / At the end of the line / You've lost that lovin' feelin / Columbus Stockade Blues / Cryin' / You don' know me / Mama don't allow / Lady blue / I Believe to my soul (a Ray Charles cover) aka Tryin' to make a fool out of me / Rhinestone cowboy
Fans of both Leon and Glen have been searching / hoping for this in studio program to be professionally released for years on DVD -- and CD. It was never released, including "Gentle", not to our knowledge. Segments occasionally show up on YouTube and appear to be from fans' recordings. Oddly, there is very little information on the net about the In Sessions series.
The above information was posted on Glen's forum on May 24, 2012 by Cowpoke, co-admin here.