GCLID stands for Google Click Identifier — a opaque string Google appends to landing-page URLs from paid ads (e.g. ?gclid=Cj0KCQ...). When a visitor lands with a GCLID, GA4 / gtag captures it as the source identifier for the session. The GCLID is essential for Google Ads conversion attribution — without it, paid traffic shows up as ‘organic’ or ‘direct’ in your reports. When migrating off GA4, you can either: (1) preserve GCLID capture in your privacy-first tool as a custom property (plausible('Visit', {props: {gclid: ...}})), or (2) decouple — let Google Ads track its own conversions via Enhanced Conversions / CAPI, and don’t try to attribute paid traffic in your privacy tool.
GCLID (Google Click Identifier)
A unique parameter Google appends to ad URLs to track clicks from Google Ads through to conversion.