It's unclear where the source files implementing gadgets go. The page only discusses defining which are available to users, but doesn't specify where the extension looks for the provided filenames for gadgets.
It should link to this: Gadget kitchen#Deploying or enabling a gadget
The definition syntax is * mygadget [options] | page names. The easiest and recommended way to create or edit the source code pages is through Special:Gadgets, where we automatically render each gadget's metadata in way that you can interact with, to create or edit the source codee page. You can learn more about the syntax and the special page, near the top of the page at Extension:Gadgets#Usage.
Alternatively, if you prefer navigate to a source page yourself, you could prepend "MediaWiki:Gadget-" in front of the specified page name. For examples of this logic, refer to the docs at Extension:Gadgets#Pages.
