Catalyzes the NADPH-dependent reduction of L-glutamate 5-phosphate into L-glutamate 5-semialdehyde and phosphate. The product spontaneously undergoes cyclization to form 1-pyrroline-5-carboxylate. L-glutamate 5-semialdehyde + NADP(+) + phosphate = H(+) + L-glutamyl 5-phosphate + NADPH Amino-acid biosynthesis; L-proline biosynthesis; L-glutamate 5-semialdehyde from L-glutamate: step 2/2. Belongs to the gamma-glutamyl phosphate reductase family.