Hi,
its not too difficult surprisingly..
Go to http://sigpac.mapa.es/fega/visor/
This will open a map. You need to zoom on the map and move to the area you are interested in.
In the top right corner of the map is a small box, move your mouse there and it opens a menu. Select Parcela from the list and the map will add red lines showing the parcelas.

Now click on Consultas on the menu bar and select Parcela (A)
Now when you move the mouse over an area it will change colour and show a box with poligono/parcela. If you then click on the one you want it'll open another window with all the information, Catestral reference etc. Print this out.

You can then go to an official estate agent or a gestor and ask them to request the 'Nota Simple' - this costs 10 euros. You could also go directly to the Registro de Propeidades office (Registre de la Propietat de Gandesa, Carrer de Ramón Berenguer IV, 27, 43780 Gandesa) or use a web based service such as https://www.registradores.org:444/propiedad/pags/faqs/faq.jsp

Hope this helps.

Also, if it is of interest, I do know of a house for sale near Gandesa.