The widgets are in "dock windows" and it looks like you undocked them
by dragging them away from the main window. The toolkit is a bit
broken on OS X and it can be difficult to put them back in.
Try dragging one of the small windows over to the main window, close
to the right edge of the main window and hold it for a few seconds. If
you see the main window change you can drop it. It is sort of drag and
drop.
Somebody suggested resetting the configuration. You can do this by
typing the following in the terminal:
rm ~/.config/gqrx/default.conf
I hope this helps!
Alex