She spent the next two seasons representing Diamonds and was ever present for Northern Superchargers in the 2022 edition of the Hundred, where she had acted as Hazell's assistant coach the previous summer.
Diamonds announced Gunn's retirement on Wednesday afternoon. James Carr, their director of cricket, said: "Before I knew Jenny personally, I had huge admiration for the international career that she had.
"It's been a huge privilege for us all to work with her and learn from her. She's produced on and off the field, and she's been one hell of an asset for us."
Smith, 27, was a key part of Diamonds' Rachael Heyhoe Flint Trophy success last year, taking 2 for 24 in her 10 overs in a tense two-run win over Vipers in the final at Lord's, but has opted to move south.
"Linsey's one hell of a competitor," Carr said. "She's an absolute firecracker who can carry people with her. She's developed her batting to be an allrounder and, with the ball, almost single-handedly dismantled the Southern Vipers' batting during the last two weeks of the summer. We're sad to see her go, but what a way to go."
Smith was part of the Vipers side that won the inaugural Kia Super League in 2016 before moving to Loughborough Lightning and then Diamonds. Adam Carty, Vipers' director of cricket, said: "It's great to have her back at the team she started her career with.
"Linsey has proven to be a tough and highly respected opponent in recent seasons, and in her we've got an experienced, proven performer who offers plenty with bat and ball whilst being a fine inner ring fielder."