Catalyzes the stepwise hydrogenation of geranylgeraniol to phytol during bacteriochlorophyll A (BchlA) biosynthesis. 3 NADP(+) + phytyl diphosphate = geranylgeranyl diphosphate + 3 H(+) + 3 NADPH Porphyrin-containing compound metabolism; bacteriochlorophyll biosynthesis (light-independent). Belongs to the geranylgeranyl reductase family. ChlP subfamily.