Over the years I have also tried to find Garret. At University in London (2004ish) I had the privilege of being in a short WFRP campaign he GMd. He also introduced me to the Game of Thrones board game and afterwards I started reading the books.
I lost touch after this and similar to yourself no luck.
It would be good to get back in touch but I suspect he may have moved away from the hobby.
If I were you, I would contact John Foody. Perhaps he still has G. Lepper's email address. In the remote past both must have collaborated extensively as G. Lepper used to publish in "Warpstone".