The 32-year-old Doumit could share time with Evan Gattis, a rookie last season who is McCann's projected replacement as Atlanta's primary starter.
After playing his first seven seasons with Pittsburgh, Doumit spent two years in Minnesota. He hit .247 with 14 homers and 55 RBIs this year, when he started only 43 games at catcher and also played in the outfield.
The switch-hitter batted .318 with 15 homers and 69 RBIs in 2008 for the Pirates and had his best power numbers in 2012, when he hit .275 with 18 homers and 75 RBIs for the Twins.
Gilmartin, the 28th overall pick in the 2011 amateur draft, was 3-8 with a 5.74 ERA at Triple-A Gwinnett last season.
Gilmartin will be at spring training with a minor league contract. The 23-year-old is 12-20 with a 4.23 ERA in three minor league seasons.