Catalyzes the carboxylation of oleanolic acid at the C-23 position to form gypsogenic acid. Involved in the hemolytic saponin biosynthetic pathway. 3 O2 + oleanolate + 3 reduced [NADPH--hemoprotein reductase] = gypsogenate + 4 H(+) + 4 H2O + 3 oxidized [NADPH--hemoprotein reductase] Belongs to the cytochrome P450 family.