Recent advances in commonsense reasoning depend on large-scale human-annotated training sets to achieve peak performance. However, manual curation of training sets is expensive and has been shown to introduce annotation artifacts that neural models can readily exploit and overfit to. We propose a novel generative data augmentation technique, G-DAUGc, that aims to achieve more accurate and robust learning in a low-resource setting. Our approach generates synthetic examples using pretrained language models, and selects the most informative and diverse set of examples for data augmentation. On experiments with multiple commonsense reasoning benchmarks, G-DAUGc consistently outperforms existing data augmentation methods based on back-translation, establishing a new state-of-the-art on WINOGRANDE, CODAH, and COMMONSENSEQA, and also enhances out-of-distribution generalization, proving to be more robust against adversaries or perturbations. Our analysis demonstrates that G-DAUGc produces a diverse set of fluent training examples, and that its selection and training approaches are important for performance.