I am trying to create a map of Indonesia, but I am still having trouble finding a grid-based dataset (example.grd), where can I download this data?
Do you want coastline data, topography data, or something else for your map?
topography data
What did you search?
@Joaquim is gently pointing you to the GMT documentation - it’s very good.
Have a look here: https://docs.generic-mapping-tools.org/dev/datasets/remote-data.html and here: Remote datasets — Remote Datasets documentation
And also the tutorials. It’s a bit annoying that we have to not only write documentation, examples but also deliver them in plates to the users.