Typically for this type of device, it is not recommended to use any type of extension cord. If you must use one due to placement for safety, use one rated for the current required by the fireplace. This should be marked on a plate on the back. If in doubt, use a cord with a higher rating. In other words, if this is rated to use 15 Amps of power, I would recommend at least a 14 gauge extension. Better still would be a 12 gauge.
Keep the extension as short as possible too.
Check with the local electrical inspector as well. Sometimes, insurance won't cover a loss if something is done counter to standard practice or recommendation.
Dan