User blog comment:Dereviewer/Future Thoughts/@comment-24685871-20151015231158/@comment-27159736-20151017173802

For my explanation I am gonna call the civillians survivors because of the apocalypse

Gideon may have escaped due to Bill blowing up the prison allowing him and several other prisoners to escape and he may have established himself as the leader of crazed survivors and prisoners.

Or Gideon may join Dipper and Ford to save Mabel due to his crush on her and or he believes that he should be the only one to control the town and to not have it destroyed by Bill.

Or Gideon is being manipulated by Bill by being told that Bill is still under his control or led to believe that he will be the god of the new world so he joined Bills army to kill Dipper and Ford.