Gately "tragically died" on Saturday while visiting the Spanish island of Mallorca, the band said in a brief statement. It was not immediately clear how he died.
"At present we don't have too many details. The rest of the boys will be traveling to Majorca later this morning," the statement said.
Gately and his partner Andrew Cowles were in Mallorca together, the statement said. They were wed in a civil union in 2006.
Boyzone was a U.K. hitmaker in the 1990s and announced a comeback tour at the end of last year. Gately also had released several solo singles and appeared in stage musicals, including "Joseph and the Amazing Technicolor Dreamcoat."
The group's manager Louis Walsh told the News of the World, "We're all completely devastated."
Elton John said he and partner David Furnish were "stunned by this tragedy."
"Stephen was the kindest, gentlest soul. We send our love and condolences to his partner Andy and to all his friends everywhere," John said in a statement.
Boyzone was one of the biggest acts to come out of Ireland in the 1990s. Former Irish Prime Minister Bertie Ahern said Gately's death was "a huge tragedy to Irish entertainment, Irish music and further afield as well."
"Boyzone and Stephen, they've all been part of Irish life and far wider than that, the last 15 years, and so successful, so it's a huge, huge tragedy. It's so sad," Ahern said.