Is it possible to convert the shapefile to a text or DAT format in GMT?
you can plot shapefile in gmt directly using e.g. gmt plot
You can use the GDAL utility ogr2ogr
to convert a shapefile to a GMT-format text file if the shapefile has points or lines. I am not sure if you can convert polygons.
You could do (which I think use ogr2ogr
under the hood)
gmt convert file.shp > file.txt