First, when you load the data using shp2gpsql-gui, ensure that you've set the SRID on the data correctly: you have determined your data are EPSG:3763, so use that number in the SRID field of the GUI.
Now that your data is correctly loaded, you can get back geographic coordinate by using the
ST_Transform function to convert them from their local system into a longitude/latitude system, like
SELECT ST_AsText(ST_Transform(geom, 4326)) FROM mytable
(4326 is the most common EPSG number used for geographic coordinates, and it's probably what you want for everyday use.)
As a side note, we can confirm that your SRID number guess is correct by running a quick transformation using your example coordinates:
Looks like it's in Portugal, so great!