Hey! 
I launched the Game-Editor2 project earlier this year - It is a complete rewrite of an old, popular for its time game engine called Game-Editor.
I chose GTK+ for the editor UI, editor-side rendering, and networking and I already have snapshots of the work in progress.
The website is: https://www.game-editor2.com/
Moreover, I just finished with the design of the showcase sites of my software from the past 7 years, which I will also release for free upon reaching certain supporter milestones and there are already some stuff released.
Many of those programs are also done with GTK+ and C.
The website is: Collection
If anyone is interested in one of the projects, please keep in mind that those projects are initially designed for Windows and shall be amended to work under Linux and other platforms. Provided that you can and want to assist, I am open to work in that direction. 