The discovery of “transitional” disks around young stars has provided the opportunity to study planet formation in situ when giant planet formation and growth are most vigorous. We know young circumstellar disks disappear on a few million year timescale and we see strong spatial variations in dust and gas around such young stars, presumably directly related to the planet formation process. Some disks show very little gas and dust within ~30 AU while maintaining an optically-thick outer disk; others show signs of an optically-thin gap separating hot gas and dust from a cold reservoir outside. Using the Gemini Planet Imager (GPI) in differential polarimetry mode, we propose to characterize a representative sample of young disks using polarized scattered light emitted between 20-150 AU from the central star. The diffraction-limited resolution of Gemini South will allow disk features to be imaged with ~6 AU resolution for the first time, including gaps, cavities, puffed-up dust “walls,” asymmetries or spiral density waves. Our proposed survey (2 nights/semester for 4 semesters) of 30 young disks will encompass the fundamental stages of early planet formation, and will be a lasting legacy for Gemini that will spur follow-up ALMA observations and hydrodynamical modeling.