Probably, I'd expect. Derivative works under more restrictive licenses are usually fine (as long as this isn't explicitly disallowed) - it's making a derivative work under a less restrictive license where you're more likely to run into issues.
From the CC-BY-SA license[1]: "Share Alike — If you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one." The GPL is probably considered sufficiently dissimilar as to make relicensing under it illegal, but IANAL either, so who knows.
"...probably considered sufficiently dissimilar..."
Probably dissimilar in what way? I see them as legally similar. The only difference I see is GPL says 'this License" where CC-BY-SA says 'same or similar license.' GPL looks similar, ergo, GPL is fine.
This is not to say that literary works should be distributed under the GPL- the GPL is intended for computer programs. Perhaps the creator of this repository misunderstands this. Perhaps the repo contains computer code that performs the transformation and his intent is to place the computer code under GPL.
Regardless of these other issues, I still do not see a fundamental difference that prevents a derivative of a CC-BY-SA work being placed under GPL.
Both licenses say "You can reproduce subject to these few restrictions, one of which is that you can't add any other restrictions to derivative works", but the "these few restrictions" is slightly different between the two licences.
