Is this a gas valve behind stove? I can't move it to turn it off
Yes it's a gas valve. Sometimes they become very stiff when they aren't operated for a long time. In the photo you posted, notice the little tab sticking down from the silver lever. That's a stopper; it limits the rotation of the lever. Make sure you're attempting to rotate the lever in the correct direction, ie not pushing against that limiter (or another less visible one).
The shutoff valve at an appliance should work reliably so that gas can be shut off in case of emergency (or convenience, as in the case of replacing an appliance). I suggest the following:

Learn where your gas meter and main shutoff are located and how to operate that shutoff.
Be prepared to replace this appliance shutoff valve - have parts and tools on hand, or a plumber identified, and work at a time when you can deal with having a gas outage for a few hours at least.
Attempt to turn the lever on the valve with a wrench. If it turns easy, operates properly, and doesn't start leaking out the valve stem then you're all done. If it breaks, leaks, or things otherwise go badly then at least you went into it prepared with a recovery plan!