Catalyzes the transfer of galactose from UDP-alpha-D-galactose to lactosylceramide/beta-D-galactosyl-(1->4)-beta-D-glucosyl-(1<->1)-ceramide to produce globotriaosylceramide/globoside Gb3Cer. Also able to transfer galactose to galactosylceramide/beta-D-Gal-(1<->1')-Cer. Globoside Gb3Cer is a glycosphingolipid of the globo serie, one of the major types of neutral root structures of glycosphingolipids, that constitute a significant portion of mammalian cell membranes (Probable). Globotriaosylceramide/globoside Gb3Cer in blood and tissue cell membranes is the antigen Pk of blood histogroup P .