Converts the D-glycero-alpha-D-manno-heptose 1,7-bisphosphate intermediate into D-glycero-alpha-D-manno-heptose 1-phosphate by removing the phosphate group at the C-7 position. D-glycero-alpha-D-manno-heptose 1,7-bisphosphate + H2O = D-glycero-alpha-D-manno-heptose 1-phosphate + phosphate Nucleotide-sugar biosynthesis; GDP-D-glycero-alpha-D-manno-heptose biosynthesis; GDP-D-glycero-alpha-D-manno-heptose from D-glycero-alpha-D-manno-heptose 7-phosphate: step 2/3. Belongs to the GmhB family.