SUMMARY


STEPS TO REPRODUCE
1. Set KDE default font to Noto Sans Condensed.
2. See that this does not apply to GTK2 nor GTK3 apps.
3. Tweak various GTK config files to set Noto Sans Condensed as the default font well.
4. Log out and back in.

OBSERVED RESULT
GTK config files are reset to "Noto Sans", nont condensed.

EXPECTED RESULT
GTK config files are either left untouched or correctly synchronised.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION
For GTK2 apps, using gtk-chtheme and gtk-theme-switch2 works, but I had to go back to KDE settings GUI after using these tools and reapply the theme to overcome some residual glitches.

The big problem was with GTK3 apps.

Not sure which GTK3 config files actually need to be set. I did :

    update ~/.config/xsettingsd/xsettingsd.conf with Gtk/FontName "Noto Sans Condensed,  11"
    update ~/.config/gtk-3.0/settings.ini with gtk-font-name=Noto Sans Condensed,  11
    in dconf, under org.gnome.desktop.interface, set font-name to 'Noto Sans Condensed,  11'.

These files and dconf settings are reset to "Noto Sans, 11" at login.

KDE refers to the font as "Noto Sans" with the "Condensed" attribute while in GTK one needs to have "Noto Sans Condensed" as the font name ; maybe the setting transfer is buggy. Or maybe this has nothing to do with my wished font being Condensed.

Anyway, I had to disable the Plasma GTKd background service to make my GTK font setting stay.