Catalyzes the decarboxylation of 3-keto-L-gulonate-6-P into L-xylulose-5-P. Is involved in the anaerobic L-ascorbate utilization (By similarity). 3-dehydro-L-gulonate 6-phosphate + H(+) = CO2 + L-xylulose 5-phosphate Binds 1 Mg(2+) ion per subunit. Cofactor degradation; L-ascorbate degradation; D-xylulose 5-phosphate from L-ascorbate: step 2/4. Belongs to the HPS/KGPDC family. KGPDC subfamily.