I'm not a user, I promise you

 Gadget was a young programmer, who in an alternate timeline, ended up on Flynn's isolated system. He was employed at Encom, and happened to be a big fan of Flynn and his work as a programmer and game designer. On occasion, he stalked Flynn to the arcade, wondering what it was that he worked on outside Encom that was so important. The night of Kevin's disappearance, Gadget followed him inside the Arcade and down to his office in the basement. He followed Flynn onto the Grid, but ended up lost on the Grid. 

When Clu took over the Grid, Gadget altered his own digital code so that it mimicked his guard programs and took on the name of Gadget. He pretended to work under Clu at first, it being a charade. He had little understanding of what was going on, but knew enough that Clu was fighting against Flynn. Gadget tried to sabotage some of the operations from the inside, but the more time he spent as a member of the Guard, masquerading as a program, the more he began to believe he was one of them. 

Now, Gadget believes he is a program, and his allegiance is with Clu.