In late summer, after flowering has finished, cut back the very long shoots by up to one-third and reduce sideshoots by two-thirds. Always prune dead, damaged and diseased wood first, and always prune to just above a healthy, outward facing bud with sharp, clean secatures.
For renewal (every 1-3 years) cut out one or two of the oldest stems down at ground level.
Some old and neglected gallicas will respond well if pruned hard in the spring, with all but the most healthy, vigorous stems being cut out completely.
Whenever you are pruning roses it is a good idea to wear protecive gloves as the thorns can cut and scrape you easily.