Gross Performance is what is required for certification purposes.It is the average performance expected from a group/fleet of aircraft.
Net Performance is the performance obtained by reducing the gross performance
in specified manner to allow for degradation that cannot be accounted for,due to factors such as difference in operating techniques,due to age-ing of the acft and different climatic conditions at different places.
Net T/O flight path is obtained by reducing the gross climb gradient by 0.8%.
It is used for obstacle clearance purposes.
A much better explanation is here
Gross vs. Net takeoff performance [Archive] - PPRuNe Forums
AF.