However, the key takeaway from this article is that Google Tag Manager doesn’t store cookies on the devices of your website’s visitors on its own, and it can function with or without cookies.
Can Google Tag Manager Read the Value of Cookies?
Yes, Google Tag Manager can read and ingest the value of any first-party cookie saved by your website (or domain) on visitors’ devices. However, it cannot read or ingest the value of third-party cookies saved by other websites (or domains).
To ingest the value of a first-party cookie in Google Tag Manager, create a “1st Party Cookie” user-defined variable with that cookie’s name, then reference the variable in your GTM container’s setup as needed.