It's basically source available with rights to modify and redistribute for non-commercial purposes and without removing the ability to pay for the app.
Custom license that doesn't meet the FSF's definition. Tldr restrictions on redistribution and minor restrictions on modification. It isn't on fdroid's main, but they host a fdroid compatible one with a out of date version of Grayjay
It's not on F-Droid, they have their custom F-Droid repository but that doesn't really mean anything. Anyone can put any app in a custom F-Droid repo, it doesn't have any meaning in regard to FOSS licensing or user freedom.