Visual Studio for Mac's shell code is written in gtk+ 2. We have supporting code in gtk+ to allow mixing gtk widgets with native widgets.
But the glue code isn't used here, because there is another interesting mechanism at play. The root window created by gtk on macOS is actually an NSWindow (see GdkQuartzWindow), thus you can use Cocoa APIs on the window. We just set the title visibility to hidden and set the native titlebar widget.
That's not the opinion of everyone. Gedit for example has an up to date Windows version. Inkscape and GIMP both have macOS and Windows versions, but I think they still run version 2.
GNOME definitely is the focus as their resources are quite limited. That's why I welcome Microsoft using it and hope to see some improvements for Windows and macOS in the future.
But the glue code isn't used here, because there is another interesting mechanism at play. The root window created by gtk on macOS is actually an NSWindow (see GdkQuartzWindow), thus you can use Cocoa APIs on the window. We just set the title visibility to hidden and set the native titlebar widget.