Right, and I'm running XFCE with a different theme, or i3 without any titlebars, and every GTK3 app that uses client-side window decorations looks out of place.

I'm not running Gnome 3, I don't ever intend to run Gnome 3, and GTK shouldn't poison the last 15 years of GTK applications to try to bump up the Gnome 3 adoption numbers by attrition.

It's petty and obnoxious.

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