The 27-year-old will join the West Country outfit at the end of the season.
"Our search for second-rows has been detailed," Gloucester rugby director Nigel Davies explained. "We are delighted to have secured Mariano's services, as his qualities will complement the players we already have in place and give us some genuine options in the second row.
"I believe this gives us plenty of strength in depth in a hugely important area of the squad as we seek to establish a powerful forward platform for next season.
"Mariano is an experienced, physical operator and his signing is another statement of intent for us."
Galarza has won 18 caps for Argentina and was part of their 2011 World Cup squad.