1. the gtk status icon is a notification, it doesn't control anything, and it was deprecated back in 3.14 and Geany needs 3.24 2. the panel/systray icons are part of the window manager functionality, not under the control of the application, and controls like "iconify" are usually added to the window border by that. 3. the most a GTK application can do is _ask_ that the window be iconified, its up to the window manager to do it
Whilst anything can be coded if the WM supports it, there are lots of Linux WMs and Windows and Macos and Geany is not likely to include anything that needs specific code for each of those.