Catalyzes the reduction of geranylgeranyl diphosphate to phytyl diphosphate, providing phytol for both tocopherol and chlorophyll synthesis. 3 NADP(+) + phytyl diphosphate = geranylgeranyl diphosphate + 3 H(+) + 3 NADPH Porphyrin-containing compound metabolism; chlorophyll biosynthesis. Cofactor biosynthesis; tocopherol biosynthesis. Belongs to the geranylgeranyl reductase family. ChlP subfamily.