Can I use an existing masonry chimney as the vent for my gas fireplace?

No. The larger size of a masonry chimney is unsuitable for use with a factory built fireplace. We do have several models of wood burning or gas burning inserts that are designed for retrofit into an existing masonry fireplace, and they come with special venting that can be used inside an existing chimney. Your local dealer can help you pick the insert that best fits your needs.