How to repair gas fireplace
1. Check if the pilot fire could be lit. Open the lower panel of your gas fireplace, you will see the gas valve and the operation instruction on the metal plate. Follow the exact procedure to light the pilot fire. For example, turn on the gas pipeline, rotate the gas valve to the pilot position and hold it down, push down the spark in the same time. (If you can find the manual, follow the instruction in the manual) (please check if your gas pipeline to gas fireplace is on. That switch sometime is located close to the gas valve and sometime is located close to your furnace in the basement or the first floor).
2. If the pilot fire is OK and stable, then turn on the gas fireplace switch on the wall. If not firing, you may have problem in either pilot firing, gas valve, or switch. Normally, use the multimeter to measure the voltage generated by pilot fire and see if it is larger than 500mV. If more, pilot firing is OK. If not, you have to clear pilot firing and reposition it so that enough voltage is generated. After the pilot firing is OK, check the switch wiring and see if it is loose. Most problems are from both places.
3. If you can not handle it, hire the professionals. Normal price is the $80-100 for the first diagnosis visit only. The parts and repair labor will be extra.
p.S.: I am not a professional but I had the same problem and read a lot from the internet and fixed the problem (loose wire in the wall switch so that the contact resistance is too high. The result is that even though the voltage generated is high enough but the allocated voltage on the gas valve is so little because the swatch has the votage fall too more.) Anyway, hope you are lucky too.