I'm the GitJournal author.
I'd be open to adding the zim wiki syntax support, assuming I can have at least one designated person who uses Zim, and can test things out and report issues. GitJournal now has basic support for OrgMode, and the code is now in a state where it doesn't necessarily assume Markdown.
How about you file an issue on GitHub? [0]. Ideally, if you could get some other people to vote on it, I'll be more motivated to prioritize it.
Additionally, do you know a good source for the zim syntax? I see [1] and [2], but I'm not sure if I'm missing more.
[0] - https://github.com/GitJournal/GitJournal/issues