I need to get sitecore information that collected from anonymous users to give him availability to export it or opt out - [GDPR]
any idea about contact ID for anonymous !
The way of doing it is dependent on the sitecore version.
Regarding Anonymous user - If the user is really anonymous, then you done need to worry about the GDPR (my view). But sometimes we map user email and sensitive personal info to anonymous user by using forms or WFFM. You can use email address of that user to query xDB (Contact Identifiers) to get the contact and contactID. Then reset informations.
Also: please note that based on WFFFM save action config, anonymous user will store in Core DB and Contact List.