During the 2012–13 season with
Neptūnas Klaipėda, Gailius averaged a career high of 17.0 points, 5.6 rebounds and 2.1 assists per game in the
Lithuanian League. For the 2013–14 season he played with the Slovenian team
Union Olimpija Ljubljana. In 16 EuroCup games played, Gailius averaged 16.1 points, 5.1 rebounds and 1.8 assists per game. In August 2014, he returned to Neptūnas, signing a one-year deal. In June 2015, he signed a two-year deal with
Lietuvos rytas Vilnius. The seasons with Rytas were two of the worst in club history, finishing in only 3rd place in the Lithuanian championship, though in 2017, Gailius sealed the win against his former club Neptūnas. In August 2017, he signed a one-year deal with
San Pablo Burgos of the
Liga ACB. Gailius spent the 2019–20 season with
Neptūnas, averaging 16.6 points and 2.9 assists per game. On September 1, 2020, he signed with
Baskets Bonn of the
Basketball Bundesliga. In 19 games played for the German side, Gailius averaged 11.1 points, 2.9 rebounds and 1.5 assists per game. On March 28, 2021, Gailius signed with
Neptūnas to return one more time to the club. On August 15, 2025, he signed with
BC Gargždai of the
Lithuanian Basketball League. ==International career==