GTK applications use a different file dialog than the kde file dialog,but I don't like that,so I enabled GTK_USE_PORTAL=1,cause that should apply the kde file dialog for all applications. But that it doesn't work,if I put GTK_USE_PORTAL=1 in .profile and re login,then the GTK apps stile use the GTK file dialog.
Each GTK applications needs to support this by using the GtkFileChooserNative API (https://developer.gnome.org/gtk3/stable/gtk3-GtkFileChooserNative.html). If they support that, they they will respond to the GTK_USE_PORTAL=1 environment variable; if they do not, then they won't, and your only option is to ask the developers of the app in question to the right API.