Gloucester have endured a disappointing 2025-26 so far, winning just two of 13 Prem matches to leave them eighth in the table, including a heavy 53-12 loss in their most recent outing, against Bristol.
Tomkins has been appointed a week after confirming his departure as head coach of French Super League outfit Catalans Dragons.
The 39-year-old was at the helm for nine months but cited personal reasons and the difficulty of living away from home among his reasons for leaving.
The French side have won four and lost three of their Super League games this season and were knocked out of the Challenge Cup in the quarter-finals by St Helens.
Tomkins is the older brother of former Super League Man of Steel Sam and has switched between the two codes throughout his career on and off the pitch.
As a player, he made more than 200 appearances for Wigan Warriors, winning two Super League Grand Finals, the Challenge Cup and World Club Challenge. He also won three England caps and played for Saracens during a spell in union before retiring in 2021 with Les Dracs.
Former back row Deacon, meanwhile, returns to Kingsholm, where he spent two seasons as a player during his career.
Since retiring in 2015 he has worked at Leicester Tigers, where he has coached the line-out, defence and breakdown, and had a spell in charge of their Prem Cup side.
“I feel like I’m ready to take on the next challenge in my career,” Deacon said.
“There’s a real ambition at Kingsholm and I’m looking forward to playing my part in helping the club move forward.”
