Gas Logs are available in two fuel types, Natural Gas and Liquid Propane (LP). You may already have a line installed in the fireplace, in which case the gas you burn will be decided.
No gas line to the fireplace yet? Check to see which fuel source your appliances currently use. If you have a natural gas heater, dryer, stove, or other appliance, then natural gas is the likely choice. If you have appliances that use propane, then select a gas log set that uses liquid propane. If your home does not have plumbing for either gas type, then check locally to determine which fuel would be best for your area.