You can do it yourself if you have confidence in your ability to change out a gas part.
Turn off the gas supply and unplug the dryer. Open the top lid and remove the front panel to access the gas valve. Mark your wires for re-connect. Remove the wires.Remove the gas valve mounting screws and unscrew the gas valve. Use pipe sealant on the pipe threads for the new valve. Tighten it securely,replace the wires and turn on the gas supply. Use a soapy solution at the pipe thread joint to check for gas leaks. No leaks,plug it in and turn it on. It may take a couple attempts to light while purging air from the line.
If you have any doubts,call a technician.