CLA's are imposed by companies, not licenses. I'm quire sure that companies with OSS projects under GPL licenses would still want external contributors to certify that the code they contribute is theirs to contribute and can legally be licensed as GPL.

Indeed, there is the Developer Certificate of Origin for that:

https://developercertificate.org/

Sure there is: https://www.gnu.org/licenses/why-assign.html

The owner of a GPL project doesn't own the copyright for contributed code (and therefore can't license it themselves under GPL), unless they receive a copyright assignment.