Galactosyltransferase involved in the biosynthesis of the bilayer-forming membrane lipid alpha-galactosyl-glucosyldiacylglycerol which is involved in maintaining constant nonbilayer/bilayer conditions (curvature packing stress). Also involved in the beta-lactam resistance. Catalyzes the transfer of a galactosyl residue from UDP-Gal to alpha-glucosyl-DAG (1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol) acceptor to form the corresponding galactosyl-glycosyl-DAG product (3-O-alpha-(D-galactopyranosyl-alpha-(1->2)-D-glucopyranosyl)-1,2-diacyl-sn-glycerol). It can only use UDP-Gal as sugar donor and alpha-glucosyl-DAG is the preferred sugar acceptor. a 1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol + UDP-alpha-D-galactose = a 1,2-diacyl-3-O-[alpha-D-galactopyranosyl-(1->2)-alpha-D-glucopyranosyl]-sn-glycerol + H(+) + UDP Activated by the negatively charged lipid phosphatidylglycerol (PG). Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.