First of all, if the documentation is not clear, please file an issue, or create a topic on Discours asking for clarification. Generic topics like “the documentation is not good” do not make it easy for anybody to understand what is your level of understanding, what it is that is not clear, or how to help you.
If you’re looking for code examples to copy and paste in your own project, you probably won’t find any; there’s the gtk-demo application that is shipped by GTK itself, and that you can use to see common usage patterns, as well as their source code. Many distributions have a package for it, like gtk-3-examples in Debian and Ubuntu, or gtk3-devel in Fedora. You should install it, and look at the various demos.
There’s also a whole pinned thread about useful documentation for GTK on this Discourse instance, with many links that you can check out.