Catalyzes the synthesis of GMP from XMP. ATP + H2O + L-glutamine + XMP = AMP + diphosphate + GMP + 2 H(+) + L-glutamate kcat is 2.3 sec(-1) for XMP. kcat is 2.39 sec(-1) for ATP. kcat is 3.05 sec(-1) for glutamine. Optimum pH is 7.5. Optimum temperature is 40 degrees Celsius. Purine metabolism; GMP biosynthesis; GMP from XMP (L-Gln route): step 1/1. Homodimer.