The enzyme, characterized from the Gram-negative bacterium Paraburkholderia rhizoxinica, participates in the biosynthesis of 3PG-factor 420. The enzyme can also accept 2-phospho-L-lactate and phosphoenolpyruvate, but activity is much higher with 3-phospho-D-glycerate. cf. EC 2.7.7.68, 2-phospho-L-lactate guanylyltransferase and EC 2.7.7.105, phosphoenolpyruvate guanylyltransferase.