Well, quick and dirty theoretical and untested idea, for poor people without production shell access:

• Assuming the MediaWiki: namespace is always 8 (if it is not, got to query first via API:Siteinfo)
• Get a list of all MediaWiki:Gadget-* pages on a site: curl -s "https://xx.wikipedia.org/w/api.php?action=query&list=allpages&apfrom=Gadget&apto=Gadgeu&aplimit=500&apnamespace=8&apprefix=Gadget-" | jq -r '.. |."title"? | select(. != null)' > textfile1
• Only get the actual JS pages: grep '.js' textfile1 > textfile2
• Cannot query revisions for several pages (titles) in one request. So for each page, list the authors of its revisions since 2015 (see API:Revisions)
• Install jq and iterate over each line in textfile2 via curl -s "https://xx.wikipedia.org/w/api.php?action=query&prop=revisions&titles=MediaWiki:Gadget-SomeGadgetNameFromTextfile2.js&rvprop=user&formatversion=2&rvlimit=max&rvend=20150101000000" | jq -r '.. |."user"? | select(. != null)' >> textfile3 to get the author of each edit on that gadget code since 2015.
• Run sort textfile3 | uniq -c
• Profit.

If you want to run this across several WMF sites, use the sitematrix to iterate.

This is great, thanks!
Questions: Above you write "use the sitematrix to iterate" - what does that mean in practice? For example, if I want to get results from all the French language projects, is there something simple we can add to the script to make it automatically run across the 8 French projects, so that output.txt would have 8 separate lists? (or combined, whichever is easier) And/or something we can add, to make it run across all the Wiktionaries?

Hi @Quiddity, thank you for your interest. Our new limited freemium model offer allows you to request features for a price you cannot beat! I wonder whether to accept some command line argument (like ./foo.sh "fr."), pulling the sitematrix and extracting the hostnames, and then match hostnames on this $1 argument.
Quick and dirty, you could wrap the code in a for WEBSITE in fr.wikipedia.org fr.wiktionary.org fr.wikisource.org; do ... loop.

Note to myself: AFAIK Quarry does not yet allow running cross-site queries.
https://quarry.wmflabs.org/query/17556 by halfak offers a similar query for the Module namespace, on a single wiki though:

USE enwiki_p;
SELECT rc_user_text, COUNT(*) 
FROM recentchanges_userindex 
WHERE 
    rc_namespace = 828 AND  -- Module namespace
    rc_type IN (0, 1)       -- Change is an edit of some sort
GROUP BY rc_user_text 
ORDER BY COUNT(*) DESC 
LIMIT 100;

In T190164#4116376, @Aklapper wrote:

Note to myself: AFAIK Quarry does not yet allow running cross-site queries.

That is covered in T95582: it would be useful to run the same Quarry query conveniently in several database