You can request account removal in account settings (here).
Press the DELETE ACCOUNT button to send a confirmation request to the e-mail address associated with your GOG account. After confirming the request via email, your GOG profile will be permanently deleted within 14 days. During this time, you’ll be able to log in and access your owned content. Once this period ends, the account's removal becomes irreversible.
Note that once your account has been deleted:
Learn more about deleting your GOG account here.