Emmanuele Bassi, a GNOME and GTK developer, writes about GTK internals, GPU rendering, the Vala language, and GNOME history.
HN links his 2017 post on Vala, which commenters cite as evidence GNOME developers moved away from it, and his GSK posts for GTK drawing.
It's used but some of the core gnome developers have advocated for people to stop using it.
What I can find with a quick search this morning: [1] [2]
Then again, there are also several people who still say it's worth using: [3] [4]
So maybe it's just Emmanuele Bassi advocating against it. If you look at the development pace, though, it hasn't really seen any significant development for quite a long time.
[1] https://twitter.com/ebassi/status/827482509982195712
[2] https://www.bassi.io/articles/2017/02/13/on-vala/
[3] https://blogs.gnome.org/mcatanzaro/2017/02/19/on-problems-with-vala/
[4] https://blogs.gnome.org/despinosa/2017/02/14/vala-is-not-a-programming-language/
Has Vala seen much activity recently? The last time I remember hearing about it was reading a blog post from one of the main GNOME developers (https://www.bassi.io/articles/2017/02/13/on-vala/) saying that Vala was basically being maintained by a single developer and he didn't recommend using it for new development.
Actually, quite the opposite, the GNOME folks have been moving away from it: https://www.bassi.io/articles/2017/02/13/on-vala/
I think the only Vala maintainer right now is from Elementary OS, where they use primarily Vala.
For Gtk+ this might be a good place to start:
https://blog.gtk.org/2016/06/15/drawing-in-gtk/
The API will likely change considerably in Gtk4, more here:
https://www.bassi.io/articles/2016/07/05/gsk-demystified-1/