It should be possible to have global gadgets, that are available by default on the gadget list of each WMF wiki. This is specially useful for keeping a single central copy of popular gadgets, such as hiding global notices, or HotCat, WikiMiniAtlas, etc.
It is already possible to invoke a user script in another wiki. A proposed solution is hold global gadgets in Wikimedia GitLab, and a proposed extension named Produnto will deliver it to Wikimedia projects.
Old description:
The possible repositories would be Meta-wiki or MediaWiki.org (see also T59336: Make metawiki the central OAuth wiki).
This probably depends on T16950: [Epic] Support global preferences on Wikimedia wikis to be fixed first
See also