2003 gran marquise number 6 plug has no compression but all others are fine.
Low or no compression in only one cylinder indicates usually indicates a valve or cam shaft problem. A valve could be burned, a cam lobe may have rounded off the lobe that opens the valve or a rocker arm or push rod could have broken or fallen off or a valve could be bent.
Removal of the valve cover to inspect the valve movement can confirm this. If the valves move like all the rest then you are probably looking at burned valves. Having no compression can also indicate valves that have too much carbon accumulated on the valve where the valve can't close.
Doing one more part on the compression test can confirm if the rings are malfunctioning. It's called a wet test, squirt oil into the cylinder and do the compression test again if you get compression or an improvement in compression then this confirms a problem with the rings.