x-gtk-show-hidden-files

x-gtk-show-hidden-files is a variable defined in `C source code'.
Its value is

nil

Documentation:
If non-nil, the GTK file chooser will by default show hidden files.
Note that this is just the default, there is a toggle button on the file
chooser to show or not show hidden files on a case by case basis.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.