ONLYOFFICE accuses "Euro-Office" maker Nextcloud and IONOS of License Violation
(www.onlyoffice.com)
OK this is getting a bit hairy, but AFAIK only the creator of the GPL code maintains copyright, and can take his code to a closed source project.
BUT the code that has been released as GPL remains open source, and cannot be made to be NOT open source.
So anybody has a right to use it, as long as they keep it open source. So in the case of open source projects, it's academic that the creator of the code maintain the right to use it without the GPL conditions.