2.1% is minimum certification (CS/FAR/AP-25) value. For instrumental approach according to 8168 you have to have 2.5%. If there are some significant obstacles, ACG should be bigger, than 2.5%. However MDA(H) must be povided for ACG = 2.5% as well as for bigger one. No gross/net flight path definitions for approach path as well as for other flight phases, describes in 8168. "Net" for abnormal (engine-out) only.