Degradation of inorganic polyphosphates (polyP). Releases orthophosphate processively from the ends of the polyP chain (PubMed:10382967, PubMed:26576296). Has also polyphosphate:ADP phosphotransferase activity, catalyzing the production of ATP from ADP and polyP (PubMed:26576296). [phosphate](n) + H2O = [phosphate](n-1) + H(+) + phosphate [phosphate](n) + ATP = [phosphate](n+1) + ADP Exopolyphosphatase activity is stimulated by NH(4)(+) and K(+). Phosphotransferase activity is insensitive to the addition of K(+) or NH(4)(+) ions. kcat is 57.02 sec(-1) for hydrolase activity with polyP(75) as substrate. kcat is 53.03 sec(-1) for hydrolase activity with polyP(65) as substrate. kcat is 41.23 sec(-1) for hydrolase activity with polyP(45) as substrate. kcat is 40.20 sec(-1) for hydrolase activity with polyP(25) as substrate. kcat is 3.93 sec(-1) for transferase activity with polyP(65) as substrate. kcat is 4.28 sec(-1) for transferase activity with polyP(25) as substrate. Homodimer. The catalytic activity is found in the N-terminal region formed by subdomains I and II. The peptide that connects subdomains II and III is also essential for activity. The C-terminal domain may be important for the recognition and/or interaction with long polyP chains. Belongs to the GppA/Ppx family.