Out of 100% about 90% of gross primary productivity is lost during respiration and other vital activities. GPP minus respiration losses is the net primary productivity yibnd this is available to the organism of next trophic level (herbivores and decomposers) for consumption.
Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. So, nature tend to increase gross primary productivity through supporting the large number of plant (producers) in an ecosystem.
Net primary productivity is the available biomass for the consumption by heterotrophs (human and animals). Man tries to increase net primary productivity by cultivating food and other crops to fulfil their needs.