Three dimensional dislocation dynamics (DD) simulations are performed to make a critical assessment of two classes of strain gradient plasticity theories currently available for studying size dependent plasticity at the micrometer-scale. Non-incremental version of strain gradient plasticity theories is characterized by certain stress quantities expressed in terms of increments of strains and their gradients, whereas incremental version of theories employ incremental relationships between all stress quantities and the increments of strains and their gradients. From the previous work, two classes of theories showed marked difference in the prediction of the onset of plasticity under non-proportional loading. In this work, our DD results show that plastic flow could continuously occur even with passivation, which lead to severe hardening, without showing the elastic loading gap.