We highlighted how the gut–pancreas axis (via incretins and nutrients) influences islet hormone secretion; how the gut–endocrine axis (gut–brain and gut–adipose signaling) regulates appetite and adiposity; how the gut–liver axis (through bile acids and endotoxins) contributes to NAFLD and hepatic insulin resistance; and how the gut–kidney axis (microbial toxins and hormonal crosstalk) affects diabetic kidney disease. This evidence concerns the gene GCG and diabetic kidney disease.