I used to install gas logs and the main issue we ran into was rusted old log lighters that had to be removed prior to install, I never failed with the process bellow. DISCLAMER MAKE 100% SURE THERE IS NO GAS IN THE LINE. I never had to worry about that because a lighter is just a pipe with holes in it the gas dissipates when not in use.
you need 2 pipe wrenches as Richard mentioned, good leather gloves, a propane tourch, and some ice cubes. Heat the larger nut part of the union, this will of course heat the whole union but you want to concentrate the heat on the nut. You then use the ice to cool the side of the union that receives the nut. Basically you are expanding one side with the heat and contracting the other by cooling. You then work very quickly with the wrenches. I never had to repeat the process more than 3 times before it broke loose.
