This line should definitely be capped off if you are not going to use it. Go to any hardware store and get a black iron cap. Most likely, the size is half inch. It looks a bit like a mushroom cap with no stem. It is threaded inside. You also need some pipe sealant to apply liberally to the threaded portion still in the house. If you don't own two pipe wrenches, then get some help from someone who does own them and knows how to use them.

Gas is nothing to take for granted and your concern is justified. This cap needs to be installed very tight to make sure this pipe is sealed off 100 percent.

I agree and disagree with the above post.

I agree that any flex line should be removed and saved for possible future use but there is no justifiable reason to remove any black pipe back to the shut-off valve. That pipe was installed by a licenced gasfitter and tested. It has been in place since that time without any leakage so why disturb something that poses no hazard. I simple cap plus pipe sealant is all that's required here in case someone or something accidently turns the shut-off on.

I would agree it would still be safe if a licensed gasfitter had removed the gas range & the accompanying flex line. However, if he had of been a licensed gasfitter he would have capped or plugged himself.

The person that did the work is some "Joe-off-the-street" & we don't know if he loosened some of the black pipe fittings in the process.

If the OP is not replacing the gas range w/ a gas range I personally would not like the looks of it being there so would remove it anyway back to the shut-off valve.

I'd go with either of the above but for sure remove any flex connectors. Be careful not to break anything upstream of the cut-off valve loose, and check the cap/plug for leaks with soapy water. When going to get a cap/plug, simply measure the inside diameter of the fitting needing to be capped/plugged. Cap= screws down over a threaded fitting. Plug= screws into a threaded fitting.

Everyone is assuming that it is black pipe. I dont' think the person posting said that. Possibly it's bottled gas and comes in with copper. If this is the case, most of the instructions won't help that much.

"I agree that any flex line should be removed and saved for possible future use "

Flex line for the final connection to gas appliamces is not rated for use more than once, nor is it intended to be repeatedly bent and reformed when an appliance is moved.

This is one shot stuff.
Form it, connect it, use it.
If you need to move the appliance close the shut off and disconnect the flex.
If you do not reform it you can use it over for that appliance, but other than that replace it.

If the pipe is black cap it or plug it with the correct fitting.
Copper is usually folded back and tightly crimped after the last valve.