This study investigates the processing of "wh"-dependencies in English by native speakers and advanced Mandarin Chinese-speaking learners. We examined processing at a filled gap site that was in a licit position (non-island) or located inside an island, a grammatically unlicensed position. Natives showed N400 in the non-island condition, which we take as evidence of gap prediction; no N400 emerged within the island. Learners yielded P600 in the non-island condition, suggesting learners did not predict a gap, but rather experienced syntactic integration difficulty. Like natives, learners showed no effects inside the island. Island sensitivity was also observed for both natives and learners in an offline acceptability judgment task. We also explored whether event-related potentials (ERP) responses were related to attentional control (AC), a cognitive ability that has been related to predictive processing in native speakers, in order to examine whether variability in processing in learners and native speakers is similarly explained. Results showed that increased AC was associated with larger N400s for natives and larger P600s for learners in the non-island condition, suggesting that increased AC may be related to prediction for natives and to integration effort for learners. Overall, learners demonstrated island sensitivity offline and online, suggesting that second language (L2) processing is indeed grammatically-guided. However, ERP results suggest that predictive processing in the resolution of wh-dependencies may be limited, at least for learners whose first language (L1) does not instantiate overt wh-movement.