Thanks Servicer! Will I need to turn off the gas to replace these parts? I am handy but not comfortable with changing gas valves. Do you know who sells these replacement parts?

Hello,
No, the gas will not need to be turned off, because you're just replacing the components which slide onto the two valves, not the valves themselves. Though you will need to unplug the unit or flip the breaker.

This is what the potentiometer looks like:
Part Details - BOSCH Potentiometer No longer available, part number: 189816

The hole in the center is where the valve stem slides through.

Anytime. You too have a nice holiday.

It's pretty simple on this unit.

The first thing to do is to unplug the unit or turn off its breaker.

Potentiometers:
Remove all knobs
Remove the two small screws in the bottom corners of the control panel
Remove the griddle thermostat bezel. The screw holding the bezel helps to hold the panel on.
Remove the control panel. (Remember to unplug the light)
Unplug the potentiometers and slide them off the valve stems.
Install new potentiometers.
Reinstall control panel.

Simmer Control:
Remove grates, burner caps and brass flame spreaders of the left side spill tray.
Remove the pipe nut. For this, you'll need to open up a crescent wrench wide enough that it will fit into the two notches on the pipe nut. Then you can use another wrench to turn it.
Lift burner bases, unplug electrodes then remove bases.
Lift up spill trays. (If the spill trays have never been removed there will be some double sided tape holding them down. Just keep applying force and they'll lift up.)
Remove the heat shield(s).
You should them be able to see the spark module, simmer control and solenoid valves.

Before removing the simmer control you may want to make a little wire diagram to insure it's rewired correctly. You can also go "wire-per-wire" by removing a wire from the old unit and installing it on the new unit one at a time.

Button it all up, reapply power and give it a shot.

Let me know if you have anymore questions