Gadgets don't have a way to conditionally load themselves (apart from the mostly-static evaluation based on user preferences, user rights, and current skin).

The current way that the community tends to optimise high-visibility gadgets is to either:

1. Lazy-load dependencies. Move the gadgets dependencies from being static in Gadgets-definition to dynamic in the main script via mw.loader.using(), e.g. after its main conditional check for which page to initialise on, often as part of the same callback that waits for $.ready. Something like if ( wanted ) { when( using, ready ).then( init ); }. The gadget's own code would remain unconditionally loaded, but only act on certain pages and only load heavy dependencies when needed.
2. Lazy-load the whole gadget. Split the gadget, moving both the dependencies and the gadget's own code into a separate hidden gadget, with the original gadget just a dep-free placeholder that calls mw.loader.load() from an if-statement.

See also T31272 and T63007, which have been awaiting resourcing for a few years.