Took me a while to find it, but this seems to be the repository:

* https://github.com/alainm23/planner

Some notes from a quick browse:

- Licensed GPL-3

- Written in Vala, using GTK

- Uses sqlite

- Meson+ninja as build system (which I like)

If it ran on Android too I'd replace Orgzly with this as I wish to have a tool I use both on my phone and laptop, synchronized via Syncthing (so far orgmode in emacs + orgzly gives me what I want, but I wish I had something a bit more pretty and user friendly like this Planner).