What FO data is lost by outputting GEDCOM and then re-importing?
For version 7.0 there is no data lost that is related to your database. For version 6.0 the loses include the todo list and the links to the Multimedia images. This means that if you were to export a GEDCOM file from 7.0 and import the data into 6.0 this data from the GEDCOM file would be ignored.