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
The GNU General Public License version 3 was declared a “BY-SA–Compatible License” for version 4.0 on 8 October 2015. Note that compatibility with the GPLv3 is one-way only, which means you may license your contributions to adaptations of BY-SA 4.0 materials under GPLv3, but you may not license your contributions to adaptations of GPLv3 projects under BY-SA 4.0.
With one-way compatibility to the GPL, no new obligations are imposed under the BY-SA license, either upon the original licensor or any downstream adapter who wishes to license their contributions under the GPL instead of BY-SA. However, those who adapt BY-SA works and choose to license their contributions under the GPL do, however, have to comply with the GPL obligation to distribute or make available the work in the preferred form for making modifications. If an adapter cannot provide or make available source either because she never received the work in modifiable format from the BY-SA licensor or cannot convert the content to modifiable format, then that person cannot take advantage of the one-way compatibility declaration and use the GPLv3 for her contributions.