The gauge just unscrews. Whoever installed it may have used a pipe dope that hardened. You shouldn't break anything taking it out. When installing the new one get some teflon pipe tape and wrap about 8 turns on the threads then screw it in. If the housing is non-metalic just go hand tight so the housing doesn't crack. Just snug it up at first and see if it leaks. Tighten until it doesn't leak but don't use a wrench.
Al