What is the GCR control doing?

X-Rite profiling packages offer controls for generating black from colors, including GCR (Gray Component Replacement). GCR replaces CMY builds with black to reduce total ink usage and maintain neutral gray balances. The amount of black that is applied to your image will depend on your black start point, the total amount of GCR that you use, and the amount of saturated or neutral colors in your image.

Real World Color Managment by Fraser, Murphy and Bunting is an excellent resource for additional information on black generation.