There is a way to show all gadgets, and that's by pressing WindowKey + G
But pressing that again won't hide all gadgets. There are some situations where a user want to hide all gadgets, such as reading a webpage where the content is covered up by the gadgets, or watching a video full screen and don't want any gadget on the screen.
If there is no fast way to do it, the only method will be to right click on each one of them and deselect "always on top". Then when you are done, right click each and every one of them and choose "always on top" again.