In my market area, Boston's North Shore, having a fireplace is considered an added value to a property. Whereas there seems to be a trend for gas FP in newer homes, the charm of a wood burning fireplace is more attractive to a buyer. Period homes that have fireplaces in almost every room have great appeal, but the reality is that most are not practical (bedrooms for instance) or functional. Buyers like the idea that they can convert an existing wood burning FP to gas (assuming you have gas in the house) and I have seen great examples of this. So to answer the question from a seller's perspective, converting to a gas FP will not add value.