Net production is the energy that left after the cellular respiration process while gross productivity is the total energy or the amount of total synthesized organic matter acquired at each trophic level from producers to the highest trophic level. Gross productivity also includes net productivity in it.
Moving in trophic level from one to the next leads to the decrease in the efficiency of energy conversion which means less amount of energy is move from the previous level as net productivity decreases at each level.