"It's a shame because I think they've enjoyed it. They have both done some good things for us and I think they would have got better if they'd stayed longer."
Belsonic at Custom House Square in the city centre will also feature veteran Welsh rockers Stereophonics.
Brown and Gill spent the last month at Valley Parade after being granted a temporary exit by their parent club. The duo excelled in League Two, with Gill playing in all five fixtures during that time and Brown making three appearances despite a knee injury.
"The crowd will vary: there's stuff for older audiences like the Stereophonics and Paul Weller, as well as pop acts like David Guetta and Florence."
Other headline acts include 2manydjs, Biffy Clyro and David Guetta.
