Orlandes wrote:
When raising the thermostat setting so that the heat is supposed to
come on, the gas valve sticks. I can free the sticking with vibration
by tapping in the area.

Do I need an entire gas valve assembly, or can I just replace part of
the switch? Would a service call be able to clean the valve?

TIA

First measure the voltage applied to the coil of the valve. (It could
be as simple as a bad connection at the terminals) If it is there and
tapping the valve makes it operate, the valve is bad. Replace it.

The valve is not serviceable, it needs to be replaced if bad.