gclid
gclid (Google Click Identifier) is a unique parameter Google Ads automatically appends to an ad's link on click, something like ...?gclid=Cj0KCQ.... Google uses it to match that click back to a specific campaign, keyword, and ad once it later receives a conversion signal.
Why gclid matters to a buyer
For traffic coming from Google Ads, gclid is what lets you feed conversion data back into the Google Ads account through offline conversion import or the Conversions API — without it, optimizing campaigns on real conversions instead of raw clicks isn't possible. The gclid value needs to survive the whole user journey: from the ad into the tracker, through a prelander or PWA, all the way to the conversion on the offer.
gclid alongside a postback
The usual setup: gclid enters the tracker as a macro along with the click, the tracker stores it next to its own internal click ID, and when the offer fires a conversion postback, the tracker uses that same click ID to look up the original gclid and forward it to Google. APEX supports passing arbitrary parameters through postbacks and S2S integrations, so gclid can travel the full chain without losing attribution.
FAQ
- Is gclid only relevant for Google Ads traffic?
- Yes, it's a Google Ads-specific parameter — other sources (push networks, native, Facebook) use their own click identifiers, like fbclid for Facebook.
- What happens if gclid gets dropped along the way?
- The conversion can't be fed back into Google Ads as an offline conversion, and the campaign keeps optimizing on raw clicks alone, with no visibility into actual traffic quality.
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