In order to live in Galt’s Gulch, a person had to agree not to work in his own profession and give the world the benefit of his mind.
Galt’s Gulch is Mulligan’s Valley. Galt tells Dagny that Galt’s Gulch was founded as a haven for outcasts. They all agreed that they were not going to be part of the rest of the world with their minds, even if they were physically.
When a man took our oath, it meant a single commitment: not to work in his own profession, not to give to the world the benefit of his mind. (Part 3, ch 1)
In order to live in Galt's Gulch, each man must take this oath. It ensures that they are all out for their own best interests, and that is it.
He describes them as “a voluntary association of men held together by nothing but every man's self-interest” (Part 3, ch 1). All of the residents of this secret society are "on strike" at least part of the time, practicing the professions they love for their own purposes only, and denying the rest of the world thier skill sets.