Answer:
Inattentional blindness.
Explanation:
Inattentional blindness occurs when we fail to notice something that is in our visual field because we are focussing our attention on something else. This is useful when we want to concentrate on a task, and to carry it out, we have to "ignore" all the different stimuli that are around us and that do not help with the task.
The gorilla in the study was a stimulus that wasn't important for the task, so the participants didn't pay attention to it.