In the context of the restructured competitive markets, flowgate is a simple way of saying "hub" or "commercially important bus". The following site gives the definition below:
A flowgate represents a point (or coordinated multiple points) of physical connection within the power system that represent both a monitoring point and a point of potential congestion in the system. A Commercially Significant Flowgate is being defined as a flowgate that, when constrained, has price differentials on the two sides that are of sufficient magnitude to alter the economics of commercial transactions. The precise definition will be required for each region that adopts this methodology.