Need advice on removing fireplace gas starter pipe
I had the same problem when converting mine. The pipes have been subjected to repeated extreme heat and cooling cycles and will be difficult to unthread.

Your best bet is to soak with penetrating oil (slide some cardboard under prior to spraying so you don't soak the bricks). Soak repeatedly and tap the pipes frequently to help the oil penetrate into the threads. Always use two pipe wrenches, one to "back up" the fitting to reduce stresses on the rest of the assembly. You may also find that heating the fitting with a small propane torch will expand it a bit, facilitating removal (keep a water spray bottle handy in case the residual penetrating oil flames up, and never spray oil while the lit torch is near).