There is a package available in the standard repos called
kde-gtk-config it supports both gtk2 and gtk3 install that package then after installation you will find under System Settings > Application Style > GNOME/GTK Application Style
font setting. For gtk font setting to work you have to have both gtk2 and gtk3 set depending on which the application uses.
This is a link to the arch wiki on gtk it explains most everything.
If you would like to do it the manual way and edit the gtk rc files that is also explain with examples on the arch website.
EDIT: You can also use the gsettings command in the terminal.
gsettings set org.gnome.desktop.interface monospace-font-name 'Monospace 11'
gsettings set org.gnome.desktop.interface document-font-name 'Sans 11'
gsettings set org.gnome.desktop.interface font-name 'Sans 11'
EDIT: If you have issues with scaling see the font section in the below link it explains part of the reason why and a work around.