This issue summarises current state from multiple sources.

First, #139 was merged without running tests on CI. Retrying tests shows that they do not pass on Windows, even though tests passed before that PR was merged. Tests failures in gepub test suite were somewhat hacked around in #140, but I believe this just tried to hide the issue.

1.0.16 also broke asciidoctor-epub3, so it had to lock gepub to 1.0.15 for now: asciidoctor/asciidoctor-epub3#474

I believe that the reason why file deletion fails on Windows is because file descriptor was not properly closed and is still open by the time deletion attempt happens. On Linux/macOS it is OK to delete opened files, but file descriptor leaks can cause other issues.