***
If you're not sure this is actually a bug, instead post about it at https://discuss.kde.org

If you're reporting a crash, attach a backtrace with debug symbols; see https://community.kde.org/Guidelines_and_HOWTOs/Debugging/How_to_create_useful_crash_reports
***

SUMMARY
I checked gtk settings on my machine and found that kde-gtk-config seem to not to sync anything except of gtk-font-name. But there's more to sync (description / settings.ini key / gsettings key / xsettings key):

Antialiasing (0 / 1): gtk-xft-antialias / org.gnome.desktop.interface.font-antialiasing (none / grayscale / rgba) / Xft/Antialias
Hinting (0 / 1): gtk-xft-hinting / <none> / Xft/Hinting
Hint style (hintnone / hintslight / hintmedium / hintfull): gtk-xft-hintstyle / org.gnome.desktop.interface.font-hinting / Xft/HintStyle
RGB subpixel order (rgba (none) / rgb / bgr / vrgb / vbgr): gtk-xft-rgba / org.gnome.desktop.interface.font-rgba-order / Xft/RGBA
Monospace font: <none> / org.gnome.desktop.interface.monospace-font-name / <none>
Document font (KDE doesn't seem to have this, perhaps set the general one?): <none> / org.gnome.desktop.interface.document-font-name / <none>

STEPS TO REPRODUCE
1. Check settings.ini / org.gnome.desktop.interface section in dconf-editor / dump_xsettings

OBSERVED RESULT
Font settings to be set

EXPECTED RESULT
Font settings aren't set except of gtk-font-name (general font)

SOFTWARE/OS VERSIONS
Operating System: NixOS 24.11
KDE Plasma Version: 6.0.5
KDE Frameworks Version: 6.2.0
Qt Version: 6.7.1
Kernel Version: 6.9.1-zen1 (64-bit)
Graphics Platform: Wayland
Processors: 8 × AMD Ryzen 5 3500U with Radeon Vega Mobile Gfx
Memory: 5.7 ГиБ of RAM
Graphics Processor: AMD Radeon Vega 8 Graphics