The tough scrummaging set-piece specialist, who has nine Italian caps, follows hot on the heels of his national captain Marco Bortolami, who became Ryan's first high-profile signing of the season yesterday when he agreed a two-year contract to join the Kingsholm club from Narbonne.
With Phil Vickery currently out of action because of a back injury, Ryan has moved quickly to grab Nieto - one of the most highly regarded tight forwards in Europe - to bolster his ranks ahead of next season.
"I am positive Carlos will be another great addition to our squad," Ryan told www.gloucesterrugby.co.uk. "He is a high class addition to our forward pack in an area that needed strengthening because of Phil Vickery's untimely injury.
"I am motivated to sign players who I believe will bring us a new dimension and an extra edge to our game. I am sure Carlos, as an international who is getting better all the time, can do that.
"As a club we have got on with the business of planning for next season and this signing re-enforces that. Carlos is a major addition to our squad."
The Argentinian-born prop made his Italy debut against England in the 2002 Six Nations and was recalled by Pierre Berbizier to the national set-up for the 2005 summer tour of Argentina and Australia.
The Viadana forward is now a major piece of the Italian forward arsenal and he said: "This is a very exciting move for me and I'm delighted to be joining a club of Gloucester's reputation.
"As their current position in the Guinness Premiership demonstrates, Gloucester already possess a very talented squad and I look forward to giving them further options in the front row.
"I met with Dean Ryan recently and was very impressed with the set-up and the club's ambition. I believe Gloucester will be strong contenders for domestic and European honours in the next three years and that's something I want to be part of.
"It's also nice to be moving to the same club as Marco (Bortolami) and I'm sure that together we can bring some Italian influence to Kingsholm."