How should i supposed to bake cookies in gas oven?
For baking the gas burners should be at the bottom.  You should not be able to see them.  There should be a piece of metal over them.  

For broiling the gas is at the top.  You can see the flames.  Broiling is for things like meat or toasting cheese where you put them close under the flames. 

Your oven should have a dial that says Bake Broil Clean and maybe other things.  Possibly it just has numbers which means degrees of Bake until you get to Broil at which point it turns on the top one.  

Use the bottom burner and make cookies on middle rack.  

I would be interested in seeing a photo of the dial of your oven.  You can take it with your phone and add it to the question.