With gvSIG I can transform from a coordinate reference system (EPSG:23031) to another one (EPSG:25831) using a grid-shift datum transformation (Ntv2). But when I want to do the reverse transformation (to pass from 25831 to 23031), the transformation is not applied.
I am usign the grid-shift datum transformation published by the Institut Cartografica de Catalunya, that you can find in this URL:
http://www.icc.cat/cat/content/download/20600/65373/file/Malla_NTv2_11_…
In this zip file you'll find the direct and the reverse transformation.