Is it safe to have a heating vent covered by a gas range/oven
Is this safe? Probably.
Is it a good idea? NO!
There are two problems:

Air flow to the room

A central heating/cooling system is most efficient if the air goes where it is most needed. Major appliances, built-in cabinets, etc. don't need to be heated &amp; cooled. Most of the time ductwork terminates either in the walls, ceiling (good for cooling but not for heating) or in the floor near the walls. That makes sense because if the ducts run to the middle of the floor then you severely limit how you can use a room.
However, if registers are in the wall near the floor, or in the floor near the wall, then it is important to make sure that those areas are not blocked by things like an oven or range or wardrobe or other large furniture or appliance. Even a large sofa can be a problem if pressed up against a wall.
Often there are simple solutions. Depending on the setup, you may be able to redirect the air flow with a piece plastic or metal, so that the air flows out the bottom front of the oven into the room, instead of flowing every possible direction around the oven.
For permanent fixtures - e.g., built-in cabinets - you can actually extend the ductwork so that it terminates with a register in the toe-kick space.

Effects on the Oven

If you have enough air flow that you can actually feel it around/through the burners then it will affect usage of the burners, and possibly the oven underneath the burners, depending on the design. This could result in higher gas usage than necessary and also unexpected changes (up or down) in the burner flames when the central heating system turns on and off. Definitely not a good thing.
But is it enough to kill you? Probably not. But I'll bet it is increasing your costs for heating and/or cooling and/or cooking.