KDE uses the "system" font hinting as default (where is this system hinting config located?), and GTK not. So, make KDE uses the QT configuration, which will force GTK to use the same. On KDE Settings, Application Appearence, Fonts, I selected "Use anti-aliasing" as "Enabled", opened "Configure", than unchecked "exclude range", checked "use subpixel", and select "slight" for "hinting style". This will force gtk to use the same font hinting.
