First, I'd check with your insurance provider and see what having a working fireplace does for your insurance premium. Second, if it exist, and you have a local property maintenance code and inspections, you'll have to fix it in order to be in compliance with the code--if it isn't working, it will be considered in disrepair. The alternative is to remove the gas fireplace altogether. If you don't remove it, and it isn't made operable, some cities will require the fireplace to be made inoperable by disconnecting the gas, covering the fireplace opening over and permanently closing off the chimney to avoid a tenant from using it for wood/trash burning. Third, if the market you're in makes a gas fireplace a bonus, I'd pop for fixing it and making it safe to operate.
