Converts the D-glycero-alpha-D-manno-heptose 1,7-bisphosphate (alpha-HBP) 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 kcat is 29 sec(-1) and 13 sec(-1) with alpha-HBP and beta-HBP as substrate, respectively. Thus, the enzyme displays 6-fold more efficiency towards the alpha- than the beta-anomer (PubMed:20050615). 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. Capsule biogenesis; capsule polysaccharide biosynthesis. Monomer. Belongs to the gmhB family.