Is your gas oven not powering up for some reason?
Gas and electric ovens require different electrical output levels in order to properly operate. Gas ovens run on 120v while electric ovens require 240v, a big difference. You can use a multimeter to check the electrical output at the wires leading to the outlet box where your oven is plugged in. If it's an electric issue, your 'oven repair' will require an electrician's help.
Is your gas oven not heating up?
The first part to check is the igniter itself. In cases where a gas oven won't turn on, most of the time it's a defect in this part that causes the issue. What usually happens hear is the igniter becomes worn out and doesn't act as it should. It's the responsibility of the heater to trigger the safety valve to open but if this doesn't happen the oven will fail to produce heat.
When examining the igniter you might notice that it glows but fails to produce heat. This indicator suggests the igniter is too weak to work right anymore. If this is the case, the igniter will need to be replaced to get the oven working again.
The next part to check in a gas oven that doesn't heat up properly would be the spark electrode that works with the bake / broil element. Usually there will be visible damage to the electrode tip or unit if this part is causing heat production issues.
Of course, the heating elements could also be at fault. Is either the broil or bake element not producing heat when it should? Simply test by running your oven in both bake and broil modes and see which elements heat up. If either don't come on when they are supposed to... that's your answer.
Diagnosing a problem in a gas oven is a bit different than with its electric counterpart. That is because the parts and heat production method between the two types of ovens are entirely different. With a gas oven, you have to think about the functional condition of the igniter, flame sensor, gas valve, etc. With an electric oven, the way the parts work and how to troubleshoot them is a lot less difficult to understand. In general, repairing a gas oven is costlier than fixing an electric unit and requires professional help.