Gallicas are hardy to zone 4, so their roots would not be harmed by a zone 6 winter even in a container. Unless the container is truly huge, however, they would be stunted. Gallicas like to spread outwards. I grew Charles de Mills in a large container for a few years, but I ultimately had to put it in the ground because it was too constricted and was beginning to produce fewer and fewer blooms each year.
