Sounds like possible carbon monoxide or gas poisoning.

You said the gas company came out, did they bring a sniffer?

If not, have them come back with one and have them check every inch of piping.

But it sounds more likely to be carbon monoxide to me.

Get a carbon monoxide detector or two.

Put them close to your water heater or furnace.

Those are your 2 biggest producers of CM if they run on gas.

Also check the vent pipes of those appliances for proper outflow and sealing.

Good luck and god bless.

Edit: could be black mold too as mentioned above.