How do you replace gas fireplace switch?

This depends on the switch. A lot of older fireplaces have a millivolt valve with a toggle-switch sticking out. We usually remove the toggle switch and run the 2 wires that were connected to the toggle to a remote receiver box. This way any simple remote transmitter will act like an electrical toggle switch. Most receivers will recognize hundreds of thousands of signals so this is usually very convenient and takes about 3 minutes to turn a manual switch into a remote switch.