I wrote a quick python script to export a Griffith database to Tellico format. It builds off of Mathias Monnerville's basic framework and grabs most of the Griffith fields that have comparable items in Tellico's movie collection template.
Download the script. Make sure it is executable. If you have python installed in a weird place, or if Griffith is storing its database file in a weird place, you may need to edit the first few lines in the script. Then run:
% ./griffith2tellico.py > griffith.tc
Load or import the griffith.tc file.
The export of the actors and actresses may not work depending on how you have them formatted. Griffith doesn't enforce any sort of format - it's a free-form field as far as I can tell.
My python expertise is still at a novice level, so don't pick at it too much. The script is rather fragile, I'm sure. It requires pysqlite3, but I don't know if that's part of a standard python installation or not.