Catalyzes the phosphorylation of guanosine and inosine to GMP and IMP, respectively (PubMed:9357959, PubMed:11129609). Can also use deoxyguanosine (PubMed:9357959). Shows a strong preference for guanosine (PubMed:9357959). dATP, GTP and dGTP can serve as phosphate donors (PubMed:9357959). ATP + guanosine = ADP + GMP + H(+) ATP + inosine = ADP + H(+) + IMP Manganese or cobalt, but not calcium, nickel or zinc can replace Mg(2+) but results in reduced reaction rates. Kinase activity is stimulated by pyrimidine nucleotides, especially CMP and CTP, and inhibited by AMP, ADP and GMP (PubMed:9357959). Activity is stimulated by potassium or ammonium ions (PubMed:9357959). Optimum pH is around 8.3. Purine metabolism; IMP biosynthesis via salvage pathway; IMP from inosine: step 1/1. Purine metabolism; GMP biosynthesis via salvage pathway. Homodimer. Specifically transcribed in the early exponential growth phase. Belongs to the carbohydrate kinase PfkB family.