Thomas arrived at Rodney Parade from the Scarlets in 2009, and he went on to make 65 appearances for the region, forcing his way back into the international reckoning in the process.
Dragons head coach Darren Edwards admits the loss of Thomas will be a major loss to the region.
"He was a fantastic all-round player who was great in the contact area and his playmaking skills were second to none." said Edwards.
"Gavin came to the region on the back of a serious injury and ended up having three terrific seasons here. That is testament to him as a competitor and a person.
"Gavin would always say to me, 'Don't look at the age on my passport, look at what I do on the pitch'.
"He was a fantastic player and servant for the Dragons. He was a man of few words in the changing room but he demonstrated to his teammates through his actions. He was a great influence and delivered what he said, and delivered consistently."
The retirement of Thomas leaves Lewis Evans and Darren Waters as the prime candidates for the number seven jersey.
Edwards is grateful for the experience the duo have gained from playing and training alongside Thomas and urged them to make the next step.
"I can think of no better role model for them than Gavin and last season he was a great help in our back row clinics after training," added Thomas
"Both Lewis and Darren have learnt so much from him and now they need to try and follow in his footsteps."