How to make KDE applications look native in GNOME?

First you need the KDE's "System settings" application by installing systemsettings from the Software Center.

Search the dash for System settings and launch it . Then make the following changes:

• under Style → Applications → Widget style, select GTK+
• under Colors, select the colours to match your GTK theme or download the Ambiance colour scheme .
• under Icons → Theme, select the icon theme you are using in GTK apps
• under Fonts select "Ubuntu", size 11

In Ubuntu 18.04, what worked for me was:

sudo apt-get install qt5-style-plugins

This is taken from the ArchLinux documentation. It also recommends setting the QT_QPA_PLATFORMTHEME=gtk2 environment variable, but I did not need that.