Earlier quoted context omitted.
https://developer.gimp.org/core/submit-patch/
I fell for this before. Wasted a few weeks' free time adding thumbnails to the GTK file chooser, only to find out that the library itself has bugs which they refused to fix that make it difficult to do. Now I just have a patched version installed locally. The only way to improve these projects is to change how they're managed, which you can't do with a patch.
The way to change projects is to write good code and work together with the other devs that made something you wanted to contribute to and then improve it together.