Thomas has made more than 80 appearances for the Dragons since joining them from the Scarlets in the 2006-07 season.
Thomas, who has been capped by Wales at Sevens level, said: "I've played at Kingsholm twice in the last few seasons, once in the Heineken Cup and once in the LV= Cup. Although I was with the opposition, I couldn't help but be impressed by the match-day atmosphere."
Gloucester head coach Bryan Redpath said: "Olly Morgan is obviously recovering from knee surgery and we've been using Jonny May and Tom Voyce in the full-back role since Morgs got injured.
"What Martyn will give us is another specialist full-back in the squad. He's the sort of player whose style of play should see him fit in well here and we look forward to him arriving in the summer."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.