FreeGameDev

Put simply this means you now have permission to adapt another licensor’s work under CC BY-SA 4.0 and release your contributions to the adaptation under GPLv3 (while the adaptation relies on both licenses, a reuser of the combined and remixed work need only look to the conditions of GPLv3 to satisfy the attribution and ShareAlike conditions of BY-SA 4.0).

This doesn’t mean that you should apply GPLv3 to your revised BY-SA 4.0 work — in most cases it makes sense to release adaptations under the same license as the original, even if not required (e.g., in the case of CC BY or CC0) to facilitate ongoing collaboration with the “upstream” and peer “forks”. But if your use case calls for or requires (in the case of remixing CC BY-SA 4.0 and GPLv3 material to make a single adaptation) releasing a CC BY-SA 4.0 adaptation under GPLv3, now you can: copyright in the guise of incompatible copyleft licenses is no longer a barrier to growing the part of the commons you’re working in.

Julius {l Wrote}:See announcement:
https://creativecommons.org/weblog/entry/46186

short summary lifted from the longer article linked above:

Put simply this means you now have permission to adapt another licensor’s work under CC BY-SA 4.0 and release your contributions to the adaptation under GPLv3 (while the adaptation relies on both licenses, a reuser of the combined and remixed work need only look to the conditions of GPLv3 to satisfy the attribution and ShareAlike conditions of BY-SA 4.0).

This doesn’t mean that you should apply GPLv3 to your revised BY-SA 4.0 work — in most cases it makes sense to release adaptations under the same license as the original, even if not required (e.g., in the case of CC BY or CC0) to facilitate ongoing collaboration with the “upstream” and peer “forks”. But if your use case calls for or requires (in the case of remixing CC BY-SA 4.0 and GPLv3 material to make a single adaptation) releasing a CC BY-SA 4.0 adaptation under GPLv3, now you can: copyright in the guise of incompatible copyleft licenses is no longer a barrier to growing the part of the commons you’re working in.