Gegen (Radix Puerariae Lobatae), the root of Pueraria lobata, is an edible and medicinal herb which has been used in treating diabetic symptoms in the orient for thousands of years. We present an evidence map of the efficacy and safety of Gegen and Gegen formulas (GGFs) that use Gegen as an essential herb for diabetes, and also its mechanism of actions. We comprehensively searched the ancient medical records to identify empirical evidence; conducted a systematic review (SR) based on moderate- to high-quality randomized controlled trials (RCTs) to synthesize the clinical evidence; and reviewed the possible mechanisms of its antidiabetic effects. Empirical application of Gegen in treating diabetic symptoms dated back to more than 2000 years ago. Common herbs used in RCTs that accompany with Gegen included Radix et Rhizoma Glycyrrhizae, Radix et Rhizoma Ginseng, Rhizoma Dioscoreae, Poria, and Radix Ophiopogonis. The combinations used today are consistent with their usage in ancient times. Results of the SR showed that GGFs could benefit patients with type 2 diabetes for blood glucose control. When in combination with hypoglycemic agents or insulin, GGFs enhanced the glucose-lowering effect as well as the lipid-lowering effects. Also, the incidence and the risk of adverse events (AE), especially the hypoglycemic episodes, were lower in the combination group. No serious or life-threatening AE was reported. The experimental evidence presented that Gegen and GGFs might exert and enhance the anti-diabetic effects through activation of multiple mechanisms, such as reducing insulin resistance, increasing insulin release, inhibiting glucose absorption and reabsorption, and improving insulin sensitivity, glucose uptake, and metabolism.