The Take That star visited Australia, Kenya, Jamaica and the Solomon Islands to find musicians for the track, a collaboration with Lord Andrew Lloyd Webber to celebrate The Queen's 60th year on the throne.
Speaking at a screening of accompanying documentary Gary Barlow: On Her Majesty's Service, the singer confessed: "It wasn't always nice. It was lots of times s**t, it was a bit s**t."
However, Barlow clarified that he still found the eye-opening experience "really motivating".
"Sometimes we didn't know what we were going to get when we landed in places and [often] discovered better stuff than we researched," he said.
"I've made hundreds or records. I've never done anything like this before... I found it really motivating, incredibly different. It sort of brings out a new side of you."
The X Factor judge also said that he relished the rare opportunity to visit exotic locations and interact with new people.
"It wasn't even a musical achievement for me at the end of the film," he said. "I just loved meeting the people.
"There are few times I'm able to go, whether it's abroad or in this country, and actually enjoy meeting people. I often spend most of my time avoiding people if I'm honest.
"The fact that I could go up and shake people's hand and meet them and discover about their lives, I really enjoyed it - I wouldn't do it again, but I really enjoyed it."
'Sing', which also features Gareth Malone and The Military Wives Choir, will form part of a commemorative Jubilee album by Gary Barlow & The Commonwealth Band released on May 28.
The composition will be performed at the Buckingham Palace Diamond Jubilee concert on June 4 that Barlow, who begun auditions for the latest series of The X Factor today (May 23), is curating.
Gary Barlow: On Her Majesty's Service will air June 3 at 7.30pm on BBC One.
