The producer-and-songwriter - who has also worked with the likes of Rihanna, Robbie Williams, The Weeknd, and most recently Haim and Lewis Capaldi - teamed up with the former One Direction star on four tracks with Snow Patrol's Johnny McDaid in 2015, 'Already Home', 'Coco', 'Endlessly' and '5378 Miles', which are yet to be released.
Asked why they haven't surfaced yet, Gary exclusively told BANG Showbiz: ''I wrote about four songs with Harry and another good friend of mine.
''I think they are just in the vault for now.''
The tracks didn't make it to the 'Sign of the Times' hitmaker's self-titled debut solo LP, which was released in 2017, or his recent follow-up 'Fine Line'.
Meanwhile, Gary - who is gearing up for the release of his own project, 'Love Lost Freedom Found' next month - says he would love to get into the studio with Robbie again to work on his next record.
Gary and Johnny co-wrote 'I Love My Life' with the former Take That star for his 2016 LP 'The Heavy Entertainment Show'.
Asked if they've worked together since, the 'Wonderful' singer said: ''I've not got back in the studio with Robbie, but would love to anytime. He's one of our greatest entertainers.''
'Love Lost Freedom Found' is released on February 28 via AWAL.
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