As GPP stands for the gross primary productivity, it's the total amount of energy produced in the process and the NPP is the net primary productivity. NPP is the difference between the GPP and the energy used by the producer for the process of respiration.
GPP is a continuous and ongoing process that consumes the energy as compared to the NPP that store energy and not being used. Thus, GPP can affect NPP but NPP does not affect GPP.
For example, GPP is completely being dependent on the chlorophyll content of the plant, but NPP is independent of that content of chlorophyll.
