GNOME is setup by default to use QGnomePlatform and Kvantum. Qt5 Config can also be used with Kvantum, but only one Platform Theme can be used at a time.
Edit your /etc/environment for system-wide changes or ~/.profile for user account changes.
# For Qt5 Config, use qt5ct
QT_QPA_PLATFORMTHEME="gnome"
QT_STYLE_OVERRIDE="kvantum"