Catalyzes the conversion of oxaloacetate (OAA) to phosphoenolpyruvate (PEP). GTP + oxaloacetate = CO2 + GDP + phosphoenolpyruvate Binds 1 Mn(2+) ion per subunit. Carbohydrate biosynthesis; gluconeogenesis. Monomer. Belongs to the phosphoenolpyruvate carboxykinase [GTP] family.