1/ I would like to plot the age of the seafloor. I don’t know where I could find a grd file to plot this info or if there is already a command in GMT.
2/ I also would like to use GMT to plot palaeogeography reconstruction of Gondwana. Any idea how can I do that?
Many thanks
For your first question, click Datasets in the sidebar of the main GMT documentation page and look for the link too currently available remote data sets and find the crustal ages there.
For palaeogeo, take a look at GPlates. It can generate these and export it as OGR/GMT ASCII for plotting.