Global Privacy Control (or GPC) is an internet protocol that, when enabled, allows browsers and extensions to communicate your privacy and tracking preferences to individual websites. You can read more about GPC here: https://globalprivacycontrol.org/
The GPC signal is intended to communicate a Do Not Sell request from a global privacy control, as per CCPA-REGULATIONS §999.315 for that browser or device, or, if known, the consumer. Under the GDPR, the intent of the GPC signal is to convey a general request that data controllers limit the sale or sharing of the user’s personal data to other data controllers (GDPR Articles 7 & 21). Over time, the GPC signal may be intended to communicate rights in other jurisdictions.