Gatt, who last played for Molde in the Norwegian Tippeligaen, is the sixth player to join the MLS expansion franchise from Scandinavia since sporting director Manny Lagos' scouting trip in September.
"Josh has been on the American soccer radar for years," Lagos said. "He's a young player, who left for Europe to build his career. He's had some really good moments, and a couple of tough injuries.
"But I think this situation lines up perfectly, in regards to where Josh is as a player, and what we're doing as a club."
Gatt, 25, has battled knee problems since making his U.S. debut in 2012.