I think I might have not made clear, but http://isthereanydeal.com/ not only is a deal seeker, but actually has a collection manager (which serves as a base for the deals you don't need since you already own those games). The collection can be synchronized with a user's steam ID and the games purchased in other digital stores can be imported to http://isthereanydeal.com/ through the Greasemonkey script.
I was thinking on the grounds of LaunchBox being able to import the CSV/TSV/JSON file generated by http://isthereanydeal.com/ data export so that it could be fed with its game list. Since there is already the mentioned script solution to basically centralize most of a user's PC collection at the website, I thought it would make sense to make use of its unified game list.
Surely is an extra step since a user must use ITAD in the first place, but I think there is fair share of PC gamers who already do and it still is the only website I know which can integrate so many digital store purchases into a single collection without manual addition.
Being able to directly retrieve the game lists from each store would most likely be better, but I suggest this in the hope that the data exported by ITAD can be a useful replacement for the seemingly difficult issue of importing games from a GOG library (and possibly other non-steam digital stores).
Of course this wouldn't help with the Launch part of LaunchBox, but I'm looking for a way to better organize my gaming catalog with the help of LaunchBox features, even if I have to go GOG Galaxy to actually play the game for now.