Galactinol synthase involved in the biosynthesis of raffinose family oligosaccharides (RFOs) that function as osmoprotectants. May promote plant stress tolerance (By similarity). myo-inositol + UDP-alpha-D-galactose = alpha-D-galactosyl-(1->3)-1D-myo-inositol + H(+) + UDP Belongs to the glycosyltransferase 8 family. Galactosyltransferase subfamily.