Revision 3e789697-a35c-4add-b46a-59e37a4cb994 - Open Source Stack Exchange

If you adapt¹ content licensed under CC BY-SA 3.0, you can (additionally) license your contribution under GPLv3 (but not the other way around). 

See [creativecommons.org’s *Compatible Licenses*](https://creativecommons.org/compatiblelicenses):

* CC BY-SA 3.0 is compatible with CC BY-SA 4.0.

* CC BY-SA 4.0 is compatible with GPLv3.

(¹ See my question [*What counts as adaptation for using code licensed under CC BY-SA 4.0 in software licensed under GPLv3?*](https://opensource.stackexchange.com/q/1950/138))