The municipality is bounded by the provincial capital,
Boac, to the north and east, by
Buenavista to the southeast and by the
Sibuyan Sea to the south and west. It is the second-oldest municipality in Marinduque, after Boac. Residents of Gasan are called
Gaseños. Gasan is from
Boac. The
Tres Reyes Islands off the coast of Marinduque are under the jurisdiction of Gasan, part of the municipality's Barangay Pinggan.
Barangays Gasan is politically subdivided into 25
barangays. Each barangay consists of
puroks and some have
sitios. • Antipolo • Bachao Ibaba • Bachao Ilaya • Bacong-Bacong • Bahi • Bangbang • Banot • Banuyo • Bognuyan • Cabugao • Dawis • Dili • Libtangin • Mahunig • Mangiliol • Masiga • Matandang Gasan • Pangi • Pinggan • Tabionan • Tapuyan • Tiguion • Barangay I (
Poblacion) • Barangay II (
Poblacion) • Barangay III (
Poblacion) All barangays in Gasan are bounded in part by coastline, except the mountainous barangays of Bachao Ilaya, Cabugao, Dawis, Mangiliol, Matandang Gasan, Tabionan, Tapuyan, and Tiguion, though Barangay Dawis is claiming that the coastline of Bukana, which is now controlled by Pinggan, belongs to them as well as the place itself.
Climate {{Weather box ==Demographics==