I found the problem. It seems that in the last day or so GOG changed the name an internal field on the Movies and Games page from "products" to "accountProducts". Interestingly, the wishlist data is unchanged for now. Anyway, I've fixed it on my end so I just need scrub out the in-progress features I was working on for the next version, and then upload.
Phin77: I'm seeing this as well. I only just started using Grease Monkey / Barefoot Essentials last night for the first time.. I was confused by the fact that it appeared to be an active project but one of it's prevalent features did not work. I really wish gog provided a way to share your library list with others... Would make getting duplicates as gifts on other storefronts less likely if your friends could see what you had from each store's catalog.
Thanks for delurking to post on this thread :) This is an issue which just happened to crop up shortly before you started using it - sometimes the GOG website changes and I need to adapt. I'll have a fix up soon, and Greasemonkey should download the update for you automatically once it's up.