@TdeM - following on from Rellwood's post.
In a fresh install of Portable I :
- added my file_path and file_date columns, did a restart;
- installed GFN and did a restart.
- configured GFN to use the file_path and file_date columns
- added the EPUB, MOBI and AVI books - no file paths or dates

So, I deleted the EPUB, MOBI and AVI books,
did a restart, added the 3 books - voila file paths and dates were there.
After some more tests I've concluded this: if you change GFN Configuration you need to do a restart. I have a vague feeling I might have discovered this before, mid-term memory decay is setting in. Is it possible to force/suggest a restart when Configuration is changed.
Yesterday I had to set the Date Column in Configure to do the test, but I didn't do a restart before I added the books -- hence the missing file_dates. If I add a book to my Test library today the dates are there.
BR