DO NOT use anti-seize on GP threads! Use ceramic paste on the body portion between the threads and the tip as it is here that carbons up and causes GPs to stick. Putting anti-seize on the threads results in distorted torque values: someone unfamiliar with this can end up stripping things and then you're up that creek w/o a paddle.
Deep Creep penetrating fluid (or an acetone+ATF home brew) applied before removing and after bringing the engine up to temp is the best approach to removing GPs. If a GP is really tough I will first look to apply tightening torque to help break things free BUT one might want to use a torque wrench and make sure to keep it under about 25Nm. Do NOT force things as that'll lead to a broken GP: I have a fairly rare engine that some idiot trashed the head on trying to remove a broken GP [they almost certainly broke themselves]. I NEVER have an issue with GPs that I install (because I use ceramic paste). I have a fair amount of experience given many years with diesel cars, trucks, generators and tractors.