We report on compositional tuning to create excellent field-performance of Jc in "self-doped," GdBa2Cu3O 7-y (GdBCO) coated conductors grown by ultrafast reactive co-evaporation. In order to give excess liquid and Gd2O3, the overall compositions were all Ba-poor and Cu-rich compared to GdBCO. The precise composition was found to be critical to the current carrying performance. The most copper-rich composition had an optimum self-field J c of 3.2 MAcm-2. A more Gd-rich composition had the best in-field performance because of the formation of low coherence, splayed Gd 2O3 nanoparticles, giving Jc (77 K, 1 T) of over 1 MAcm-2 and Jc (77 K, 5 T) of over 0.1MAcm -2.