This is a interesting one that I always get pushback from both home owners and gas installers.. While gas starters may be allowed in masonry systems, they are not approved for factory built fireplaces and could lead to over firing of the system and insurance issues if a fire occurred. Factory built (pre fabricated) fireplaces are a UL listed appliances and only approved parts can be used in these systems. It may be a convenient way to start a fire but it is not worth the risk!