Then you say ".......it runs like crap after the fact". How long after you bought it did it run like crap? What happened in between? If all cylinders but 1 have good compression I suspect bad valves not a bad gage. Squirt some oil in that cylinder and retest if still 0 you have valve issues in that cylinder. Might be bent, hole in piston, valves stuck open some due to lifter issues, or ?