I would not claim to be an expert, just a user of the figures and tables with a basic understanding but here goes anyway.
The gross climb performance is what the aircraft actually achieves at a given weight and the gross gradients are what it is required to achieve for certification and will have been demonstrated by test flying. Never done any but I guess that the aircraft is gradually loaded up to a weight during testing at which it just meets the minimum legal gradients with an engine out and this then becomes the base line maximum weight for that segment. I am sure there is much more to it than that but that’s the basic procedure. The gross performance is then degraded on paper by a certain amount to achieve the net performance figure and it is this performance data that is used to calculate the max. operating weights etc. This ensures that the even the worst aircraft on a fleet (old, dirty, perhaps not rigged quite right etc.) is statistically very unlikely not to meet the minimum GROSS performance that is required to comply with the regulations.
So in fact you never use gross performance, all the tables and figures are calculated on net performance. Not quite sure what you a driving at in your second question, perhaps a real expert like Alex W. (purveyor of fine performance courses) could help.