The titlebars are a design choice made by the app developer, not by GNOME. At least in the sense that it's only part of GNOME's design guidelines, app developer

The titlebars are a design choice made by the app developer, not by GNOME. At least in the sense that it's only part of GNOME's design guidelines, app developers using GTK don't have to follow it if they don't want. There are various ways you can configure/modify a GTK app to hide the titlebars and attempt to use server-side decorations. But either way you are faced with the choice of potentially hiding important functionality if the app developer has decided to put buttons in the titlebar, or having two titlebars like with this patch: https://github.com/PCMan/gtk3-nocsd https://github.com/PCMan/gtk3-nocsd