Piwigo gallery upload plugin / extension
GThumb has built-in support for publishing to Facebook, Picasa (dead) and Flickr, none of which I'm interested in publishing to because they are, as you know, third-party proprietary walled gardens.
When Picasa was shut down a couple of years ago (in favor of the silly "Google Photos" which tosses away the notion of "albums" and makes it hard to navigate), the very best solution I've found is Piwigo, which is pretty awesome and can be made to have masonry-style galleries like G+ used to have. There's also the bunch of other (old, bloated, ugly) PHP galleries that have been there since time immemorial, but I won't bore you with here because I think Piwigo stands out as something that actually makes sense. This might possibly be the reason why Shotwell has an export plugin for it (and mostly the only reason I still need to use Shotwell instead of just using GThumb).
So this is a feature request where I'm wishing for a Piwigo publishing plugin to be made in GThumb. Even Shotwell was written in Vala, maybe you can steal some of that code, there's a bunch of "piwigo" files in Shotwell's publishing plugins (there are also some other publishing plugins in shotwell-publishing-extras)
And who knows, if somehow the code is hard to interface with, maybe @plegall will be able to offer some advice ;)