Gas-flowing is also known as porting and polishing.
I did this myself using a flexible shaft (oo-er missus) and grinding stone on my Sierra a couple of years back. It doesn't really make a huge amount of difference to the performance, although it does make it smoother.
It's not a difficult job to do, just very time consuming.
Polishing is the smoothing of inlet/exhaust tracts of the head to allow a smoother passage of air into/out of the engine, as long as you are very careful and don't remove too much material, then nothing can really go wrong. The problems arise when there is only a thin layer of wall between the inlet/exhaust ports, and an oil or waterway.
Porting is when you match the inlet/exhaust ports between the head and the manifolds, to provide a smooth flow of air into/out of the engine.
Manifold gaskets rarely match the ports on the head, this is where port matching comes in. Place the gasket on the head, and locate it with a couple of studs and nuts, then have a look into the inlet ports. you will see where the gasket overlaps the port, creating a restriction. By matching the gasket to the inlet ports, (or vice versa) on both the head and the manifold, removes this restriction allowing the most air to get into the engine.
Hope this makes sense......