Another good open source, markdown based alternative that I used for a long time is Joplin. Also has git sync built in and is cross platform.

https://joplinapp.org/

Interestingly, GitJournal has an import function from Joplin.

[1]: https://gitjournal.io/docs/import/joplin/

Import your free notes that work just as well into our paid app with the same or less features.

That's a pretty cynical view of what's happening here. A real human is working full time on GitJournal.

They have built the mobile apps for you as a convenience. But if that's not a good fit for you, The project is open source[1]. Build it yourself.

[1]: https://github.com/GitJournal/GitJournal