gcs parameter?

The gcs parameter is a parameter that is part of tracking requests sent to Google Analytics 4 as part of GA4’s Consent Mode feature.
It encodes the current state of the user’s consent.

Here’s helpling.de as an example.
Before / without consent:

After / with consent:

What it means

The gcs parameter always begins with “G1” and is followed by two other digits, either 1 or 0. The G1 is a constant that indicates that the parameter is a GA4 consent mode parameter, so the first 1 doesn’t tell us anything.

The second digit indicates whether the user has given consent for the collection of personal data to be used with Google Ads. The third digit indicates whether the user has given consent for the collection of personal data to be used with Google Analytics.

gcs in Server Side GTM

If you’re looking for the parameter in your Server Side GTM, you’ll find it as x-ga-gcs in the Event Data.
That’s also the parameter name you’ll need in order to change the parameter using Server Side GTM Transformations – not gcs.
(Hat tip to Simo Ahava and George Arnaoutoglou for this)