Stéphane Lemaire, the president of the union, said guards at the jail told him the protest was a result of plans at the facility to split up members of a gang.
It's not clear whether some of the prisoners would have been moved to another facility or another part of the Hull jail.
At about 9:30 a.m. ET 16 prisoners failed to return to the cells in their wing and would not co-operating with jail officials, prompting the lockdown.
Quebec provincial police stepped in to negotiate with the inmates, who had destroyed equipment in the room including televisions, beds and furniture.
The lockdown ended peacefully less than nine hours later.
While police reported nobody was hurt, provincial police said charges may be laid.
Gatineau police were also on hand to enforce the security perimeter surrounding the jail while provincial police were inside.