GCG and type 2 diabetes mellitus: Up to now, existing drugs such as insulin and its analogues, metformin, sulfonylurea, glucagon-like peptide-1 (GLP-1) receptor agonist, dipeptidyl peptidase IV (DPP-4) inhibitor, sodium-glucose cotransporter-2 (SGLT-2) inhibitor, α-glucosidase inhibitor, etc., have been approved for the treatment of T2D.