A gas flowed head means that modifications have been made to the combustion chamber and oversizing, cleaning or gasket matching up of the inlet and exhaust ports. Exhaust ports are normally polished whereas inlet should be rough (finished with a flex hone or equivalent).
....your gas flowed question:
The modified head is then placed on a flow bench. Air is draw through each port and the flow of each is measured. Each port is then tweaked until the flow through each is the same. A difference in flow can cause power imbalance between cylinders.
All this is done to improve air/fuel flow and mix characteristics within the combustion chamber.
Porting a multicylinder head without a flow bench (gas flowing) is really the wrong thing to do although it is done as you cannot quantify what has been done and how different the performance will be.
There is much more to this than my simple description. Was involved with competition porting of heads on single cylinder motocross engines at championship level for a few years.
read it a few times in members threads but have no idea what it is. i used search but cud'nt find enough info.