in general terms, gasket match is a type of porting, where you open both ends of a matching surface to match the gasket opening.

in factory assembled engines there is often a ridge, or different shaped openings etc that decrease flow. by making bother ends of every mating surface smooth when assembled you make the engine more efficient and gain hp previously lost in manufacturing tolerances and or casting changes, etc.

basically you set the gasket on both mounting surfaces of the connection trace it out, and grind/port it open so they match perfectly. you can do this to the manifold if it needs it, to do the head you would need to remove it tear it down, and then you might as well to a full port job.

you would be surprised how far you could get in there with one of those flexible wand die grinder.