The Let It Shine judge has made a stark transformation over the past 17 years since his acting debut.
Appearing in the 150th edition of the hit ITV series, Gary played hitch-hiker Micky Shannon who was studying music and touring the country.
However, the 46-year-old Take That star has previously dubbed his acting on the show as "flatline" and revealed just why he thinks his acting wasn't up to scratch.
"It was just offered to me. 'Do you want to be in an episode of Heartbeat?' I'd never done any acting, not even in school or anything before," Gary told Live and Kicking at the time.
Let It Shine: Gary Barlow looks totally different on Heartbeat
Gary Barlow in ITV's Heartbeat
Let It Shine: Remember when Gary Barlow made his acting debut in ITV's Heartbeat?
I don't know why it was called Heartbeat because believe me, in my performance there wasn't a heartbeat, it was more like flatline.
"I fancied filling the first year of the millennium with a few challenges. The greatest thing is my favourite character is Greengrass in it.
"Every scene I do is with him, and he's like a northern comic and has got me in stitches," added Gary.
"I have to say though, if Bruce Willis asked me to do a movie with him, I'd probably think about it for three or four seconds and then give him an answer."
Later revealing that he used the same acting skills while shooting the video for Let Me Go, Gary added that he really enjoyed filming the TV show, having previously always "hated" making music videos.
Gary Barlow with the cast of ITV's Heartbeat
Gary Barlow singing while playing Micky Shannon in ITV's Heartbeat
Lulu in ITV's Heartbeat
Martin Kemp in ITV's Heartbeat
Let it Shine finalists
Tue, February 7, 2017
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Speaking on Lorraine back in 2013, Gary said: "I have to draw on my experience from Heartbeat in the 90s.
"I don't know why it was called Heartbeat because believe me, in my performance there wasn't a heartbeat, it was more like flatline."
Lulu, Martin Kemp and Gary all had guest roles in the ITV drama, and now sit alongside each other to judge on Let It Shine.