Califano will leave Agen for Kingsholm and becomes Gloucester's third international forward signing for next season with Italian duo Marco Bortolami and Carlos Nieto already recruited.
The 33-year-old, renowned as a destructive scrummager, can play on either side of the scrum and won the last of his 68 caps against Ireland in 2003.
Gloucester head coach Dean Ryan said: "Christian is a crucial addition to our squad.
"He brings with him a weight of experience that is almost unparalleled in the European game.
"He has spent the majority of his career in the Heineken Cup and will help us enormously as we test ourselves in club rugby's most challenging environment next year.
"When you are competing at the highest level you need people in your side who have been there before and proved themselves to be winners.
"Christian is one of those people and will be a massive influence at the club, both on and off the field."
Califano will be embarking on his second spell in the Guinness Premiership following a short stint at Saracens in 2002 and includes Toulouse and Auckland Blues among his former clubs.
He said: "It will also be nice to join up with my former French team-mates Patrice Collazo and Olivier Azam.
"I'm also keen to work closely with Jack Forster and pass on my experience to him. He is a young prop with a big future in the game."