Watford have released a total of 27 players from their squad following their relegation from the Premier League in the 2021-22 campaign.
In addition, the club announced on Tuesday that four first-team players had been released from Vicarage Road upon the expiration of their contracts in June.
Nkoulou featured just three times in the Premier League following his free transfer move back in October, whilst Gray spent the 2021-22 season on loan with Championship side Queens Park Rangers.
Meanwhile, Kucka has opted to trigger the break clause in his contract that allows the midfielder to end his one-year stay with the Hornets.
Elsewhere in the club, 19 Under-23s players and four members of the Under-18s squad will also depart Watford in the coming weeks, with 21-year-old Maurizio Pochettino, the son of Paris Saint-Germain boss Mauricio Pochettino, among those released.