At this prison, with the approval of the chaplain, the prison gates are opened to allow outside Muslims to also participate in Jumuah Salaat.

Answer

As far as Jumuah Salaat in prison is concerned, it is difficult for us to accept that the prison literally opens its gates to allow outside Muslims to attend. We accept that the prison allows outside imaams to come into the prison to conduct the Khutbah and Salaat, but it is difficult to accept that the gates are literally thrown open to allow just any and all Muslims of the outside public to attend. We need independent confirmation for this claim.

Assuming that what you are saying is true, and that Muslims from the street can simply walk into the prison to attend Jumuah, then according to the Hanafi Math-hab, the Jumuah will be valid. But, according to the other three Math-habs it will still not be valid. The issue of general permission is a condition only in terms of the Hanafi Math-hab.