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The Great Sumatran Fault (GSF) is a 1900-km-long fault extending from Lampung, Indonesia, to India's Andaman Islands. The fault location is not only on the land but also in the marine area. Previous studies were only focused on the land area of Sumatra and Andaman Islands even though the marine fault has also impacted earthquakes and tsunamis such as in 2004. As an effort to disaster risk mitigation, this study used the gravity method to map and study the continuity of the GSF in the marine area from the Aceh Province, Indonesia, to the Andaman Islands, India. The gravity data were obtained from Topex with a resolution of 1.85 km/px. Based on the Bouguer data, the subduction zone in the western part of the Indian Ocean is observed with the anomaly of 500–700 mGal, while the residual structure of GSF, relative to the subduction zone, only comes to clarity through a horizontal derivative transformation with anomaly 130-250 mGal. To delineate the fault's geometry, the data were inverted by GRABLOX 1.6 using Singular Value Decomposition and Occam methods. The 3D modeling results also clearly show the contrast density between regional faults such as subduction zones on the Westside of the West Andaman Fault (WAF). The GSF faults can also be well demonstrated at 50 km depth. Based on these results, the gravity Topex is potentially used as a preliminary study of the GSF activity in the marine area.