The word "Garcon" means boy in French. It was the custom on the larger plantations to have a separate house for boys once they reached their teens. The garconniere was used strictly by the family and their male visitors. It was an appendage of the house.
This is a small Garconierre on a plantation. Some plantations where there were more sons had larger houses. And, in smaller plantation houses the attic was considered the garconniere.
This had nothing to do with Slavery.