That's why its removal is "optional".
If you need it or are using it.. just leave it there.

"Ghostscript" is listed as an optional dependency in the Xournal INSTALL file.
It's a huge package, so I decided to maintain it as an optional package.

If you leave it there you will have no problems whatsoever.
However it has been replaced (or renamed) to: gs-gpl-8.54.dfsg

So that's the next package I will make available. But Xournal will never require it.