GPC is the latest attempt at allowing customers to specify how their browsing data is to be shared online, the previous attempt referred to as DNT was a relative failure.
Like with DNT, once the user specifies their preference the browser adds an additional HTTP request header:
const gpcValue = navigator.globalPrivacyControl
Additionally, websites can define that they respect the GPC request by posting a file in a file /.well-known/gpc.json
GPC is currently implemented by default in:
Brave = https://spreadprivacy.com/global-privacy-control-enabled-by-default/
In Firefox, you currently have to enable it manually:
about:config globalprivacycontrol boolean true