Extracting gravity data of oceanic region from a land-ocean data grid

Hello GMT users,
Using img2grd, I extracted satellite gravity data for a continental margin and the adjacent land.
But while imaging and contouring I want to use only the data in the oceanic region. Also I want to know the range of gravity in the oceanic region only. How can I do this ??

Please help…

For mapping is can use pscoast to set up a clip path that hides land, but for inquiring grid values over oceans only you will have to create a mask with grdlandmask and apply it to the gravity grid using grdmath.