gclid

gclid (Google Click Identifier) is added to your landing page address when someone clicks a Google ad and auto-tagging is on. GA4 reads it to show Google Ads campaign data, and Google Ads uses it to match conversions to clicks, including conversions you upload later from your own system.

On iOS traffic where Apple's rules limit gclid, Google uses gbraid and wbraid instead. These are aggregate identifiers that cannot tie a conversion to one person (Google).

If a redirect on your site drops the query string, the gclid is lost and Google Ads conversions go missing. Test with ?gclid=test on each landing page.