Back-rower Kalamafoni, 29, and flanker Ryan, 27, will be joined by Joe Ford, 26, and Jonah Holmes, 24, who have been signed from Yorkshire Carnegie.
Fly-half Ford is the brother of Leicester's England star George.
The versatile Holmes began his career at Wasps, while Ryan has made one appearance for Ireland, in 2014.
Kalamafoni has played in two World Cups for Tonga and made more than 100 appearances during his five-year stay at Gloucester.
"It is now time for a new challenge in my career and I'm delighted to be able to sign for Leicester Tigers," said Kalamafoni.
"It is obviously a big club with a lot of history and I'm looking forward to playing my part there."
Leicester Tigers have agreed the signings of Tonga international  Sione Kalamafoni from Gloucester and Ireland's Dominic Ryan from Leinster.