The thermopile in your fireplace puts out millivolts, nothing near the 120V the light switch was designed for.  It's probably just a matter of finding a switch with a low enough on resistance.  A generic low voltage switch from a electronics store, or ripped out of a toy, would probably do it (for example a 12V SPST).  Really here the smaller the better, but beware of anything too cheap as that may put you down the wrong path again.

![from discover circuits magazine][1]

There are some fireplace specific forums which could help, particularly with a model number given.  An example is http://www.hearth.com/talk/ .  The above image is from "Discover Circuits Magazine (December 2009 Issue 4)", which has a guide on this exact topic along with a clever booster solution from one "Mr. Wily Widget".  Find it over at http://www.discovercircuits.com/dc-mag/Issue_4/pg-7.htm

You can also try cleaning the thermopile which may have crust of soot, and measuring the voltage between those two exposed wires.


  [1]: https://i.sstatic.net/wHmf3.jpg