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A .gadget is just a zip folder. Download and install 7-zip Download
then right-click on the .gadget and in the context menu go to 7zip and choose extract here which should extract it to your desktop and you will have a folder put that folder in C:\Users\YOUR USER NAME\AppData\Local\Microsoft\Windows Sidebar\Gadgets
AppData is a hidden folder use Option 1 Step 3 of this tutorial File and Folder - Hide or Unhide
to unhide it.
See if it shows up in Desktop Gadget Gallery and try to run it.