Can I trick anyone into running mwscript extensions/GlobalBlocking/maintenance/PopulateCentralId.php --wiki=metawiki?
I don't have the access to do it myself.
The script goes through all rows of the globalblocks table where gb_by_central_id is null and looks up the central id via CentralIdLookup (in this case this means CentralAuthIdLookup) and updates the row. The final result is that the gb_by_central_id is fully populated.
The script was added in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GlobalBlocking/+/762493. If anyone has any functionality questions, please ask them to me.