To calibrate an oven control you must calibrate the gas control knob or the electric oven temperature knob.
To calibrate an oven you need an accurate oven thermometer. I prefer to use an OHM METER with a TEMPERATURE SENSING probe, but you have a digital temperature indicator so that will work fine.
First Turn the gas control on to a moderate oven (350degF) and allow ample time to preheat and stabalize.
When the oven burn shuts down at the desired temperature, note the temperature on your digital display and comupte the difference between the calling temp (temp set on the control knob) and the measured temp.
Grip the gas control knob and pull if straight off. (It slides on the gas control shaft. Note the flat on the shaft and the matching flat in the knob socket.)
On the back of the knob you will see a movable section that is locked into place on the knob from by a set screw. Loosen the set screw, and while holding the knob mount plate steady, turn the knob face until the desired temperature is at the top in the index position. Tighten the set screw and slide the knob back on the gas control.
Turn the gas control up about 50deg and check the display temp. You may need to repeat the calibration step a few times till you get it as accurate as you can.
Most gas controls are only truely accurate in a very small range of temps, usually about a 100 deg spread. I normally calibrate to be as accurate as possible at 325 or 350deg F which is the most often used baking temps.
Theere are many factors that can effect oven temperature, room air temps, atmospheric pressure, humidity etc, therefore you should expect a differential of 10 to 20 deg between the setting on the gas control and the oven temp when measured with a really accurate thermometer.
I would recommend calibrating at the temperature you most often require, or at the temp you most often require to be really accurate. By example, if you bake cakes or casseroles 350 would probably be best.
If you are a bread baker like me, 400 would be a better temp to set for maximum accuracy.