This review critically evaluates the current evidence regarding the effect of the dietary glycemic index (GI) on pregnancy outcomes in gestational diabetes mellitus (GDM). Current evidence, although limited, consistently supports the advantages of, and has demonstrated no disadvantages of, a low-GI diet. We conclude that pregnant women with GDM are likely to benefit from following a low-GI meal pattern, with no significant side effects, and consideration of the GI should be given when formulating a diet for GDM. However, until larger scale intervention trials are completed, an exclusive low-GI diet should not replace the current recommended diets for GDM from relevant government and health agencies. Further studies that intervene at an earlier stage of pregnancy are required.