Granbois is significant to Wide Sargasso Sea because it represents alienation, both physically and mentally. This information comes from the academic blog "Wide Sargasso Sea Setting." In the novel, Granbois represents the alienation both narrators experience. The unnamed husband feels trapped in the dense Dominican forest; he repeatedly remarks on how different this place is from his home. "Granbois" literally means big forest, exemplifying how this setting can influence and isolate someone who is unaccustomed to it. Antoinette feels mentally isolated, despite her love and appreciation for this location. She feels unloved and controlled by her overbearing husband, and later in the novel, after her husband's affair, she reveals her recently acquired hatred for the estate. This is caused solely by the mental turmoil she has sustained from her husband. Mapping Granbois on COVE exemplifies the lonely nature of the location, and how both narrators can feel alienated in differing ways. 
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Research: https://www.shmoop.com/study-guides/wide-sargasso-sea/setting.html