Global Privacy Control is a signal a browser or an extension sends on the visitor's behalf, as the Sec-GPC request header and as navigator.globalPrivacyControl on the page. The 51Did (51Degrees Identifier) ignores it.

What Ignoring It Means🔗

The usage of a 51Did comes from one of exactly two places, being an id.usage your integration states on the request or a Transparency and Consent Framework string the cloud reads a usage out of. Global Privacy Control is neither, so it sets no usage and changes none. Appendix 1 of the Model Terms for Marketing maps that framework's purposes onto the marketing usages and maps no other signal, so this one says nothing the usage values could be built from, the same position a Global Privacy Platform string is in on Global Privacy Platform and US Privacy Strings.

Three things follow.

A request carrying the Sec-GPC header and nothing else creates no 51Did, because no usage was stated and none could be worked out. The response has no fodid section at all, which is what any request with no usage gets.
A request carrying the header beside a stated usage or a consent string creates exactly the identifier that usage or string calls for. The header neither blocks it nor changes it.
The value inside the identifier is the same either way. A probabilistic 51Did is built from the device, the client IP address and the usage, and the header is none of those, so it moves nothing.

The 51Degrees client script does not read navigator.globalPrivacyControl either, and neither does the Preference Management Platform.

Acting On It Yourself🔗

A publisher who wants the signal to change what happens acts on it in their own code, before asking the cloud for an identifier. The decision is the publisher's, because what the signal means for a site depends on where that site operates and on what the site does with the identifier afterwards.

Two ways to act on it, both entirely in your hands.

Do not ask for the identifier. A request that states no usage produces no 51Did, so leaving the usage off is all that is needed. On a browser page a visitor who is not asked gives no answer, so the client script sends no usage and nothing is created.
State the usage you want. Send id.usage=non-marketing and the identifier carries that usage, which covers analytics, fraud prevention and security only. A non-marketing identifier is provided under legitimate interest, is not created under the Model Terms for Marketing and must not leave your own environment. The value is available on any resource key that carries the fodid.* properties.

Whichever you choose, the identifier records only the usage that reached the cloud. It says nothing about the signal you acted on.

Find Out More🔗

The identifier, its inputs and its usage values: 51Did (51Degrees Identifier)
Stating the usage from the visitor's answer: The Preference Management Platform Answer
Reading a usage out of a consent string: Transparency and Consent Framework Strings
What each usage permits, in the Model Terms for Marketing, version 2: https://m4ow.uk/mtm/2.txt
The Model Terms for Marketing explainer: https://m4ow.uk/mtm
Global Privacy Control, the specification and the organisation behind it: https://globalprivacycontrol.org
The W3C draft of the specification: https://w3c.github.io/gpc/