When Google Ads adds it
Google Ads appends gad_source to the destination URL at the moment someone clicks an ad, as part of the same auto-tagging step that adds gclid. It arrived later than gclid, after auto-tagging itself had already been running for years, and it rides in the same URL, from the same click, from the same platform. Nothing on your site sets it. It arrives already attached.
gad_source and gclid do different jobs
gclid identifies one specific click. Google Ads uses it to pull that click's cost, keyword, and ad group back into your conversion reporting, and your conversion import depends on it directly. gad_source does not identify an individual click. It is a short source code that categorizes roughly where or how the click originated among Google's ad surfaces. gclid is the key your reporting looks the click up by. gad_source travels alongside it as extra context, not as a substitute.
What the value looks like
In practice it tends to be a short number, for example 1. Because the value is typically just digits, case sensitivity barely applies here, unlike a long alphanumeric token such as gclid. Google has not published a full public table mapping every value to a meaning, so treat it as an internal signal rather than a code you decode yourself. Do not build reporting logic on guessed meanings for specific digits.
What actually depends on it
Both parameters travel together through any redirect that preserves the query string. If a redirect strips query parameters, you lose gad_source and gclid at the same time, not one before the other. Removing gad_source alone will not touch what the visitor sees; the page renders identically. It also will not break your Ads conversion import, since that import is keyed to gclid, not to this parameter. What you lose is extra source context inside Ads reporting itself. If a link is missing gclid entirely, that is the one to worry about. See it decoded on the gclid page, or check utm_source if the link also carries manual UTM tags.