Catalyzes the synthesis of lactosylceramide (LacCer) via the transfer of galactose from UDP-galactose to glucosylceramide (GlcCer) (PubMed:1551920, PubMed:24498430, PubMed:3099851). LacCer is the starting point in the biosynthesis of all gangliosides (membrane-bound glycosphingolipids) which play pivotal roles in the CNS including neuronal maturation and axonal and myelin formation (By similarity). {ECO:0000250|UniProtKB:Q9WVK5, ECO:0000269|PubMed:1551920, ECO:0000269|PubMed:24498430, ECO:0000269|PubMed:3099851}. This is the function of B4GALT6 (beta-1,4-galactosyltransferase 6, ENSG00000118276).