I removed a gas hot water heater. I want to cap gas line. Can I unscrew pipe and screw in doped plug without shuting main off, or is there too much pressure.

Gas lines are low pressure. While you can to it that way, why not be safe and turn off the gas at the meter, do the work and turn back on.

Yeah, shut the gas off at the meter and then, if you remove the pipe and plug the line off as quickly as you think you can then you won't even have to purge air out of the lines at the other appliances! If you do have issues at the other appliances you'll know that you not only need to purge some air from the gas lines to the appliances but that you may also have blown yourself up installing the plug... ;)

Be safe here, should only take a few seconds!

Good luck!

Mark

KA-BOOM! That's what happens if you attempt to work on a open and pressured gas line.
Take Mark and Brians advice and turn the gas off at the meter. Good luck, Tom