Transfers the glycosyl residue from UDP-Glc to the non-reducing end of alpha-1,4-glucan. [(1->4)-alpha-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + H(+) + UDP Glycan biosynthesis; glycogen biosynthesis. Forms a hetero-octamer with each protomer of the gsy-1 homotetramer bound to one molecule of gyg-1. The N-terminus is involved in interprotomer contacts with gyg-1. The interaction with gyg-1 is required for glycogen production but is not required for gsy-1 intrinsic activity. RNAi-mediated knockdown results in reduced survival. Belongs to the glycosyltransferase 3 family.