Catalyzes the decarboxylation of 3-keto-L-gulonate-6-P into L-xylulose-5-P. 3-dehydro-L-gulonate 6-phosphate + H(+) = CO2 + L-xylulose 5-phosphate Binds 1 Mg(2+) ion per subunit. Homodimer. Belongs to the HPS/KGPDC family. KGPDC subfamily.