If you look at the date of my original post and of the one above, this is how long it took me to decide.
The way I see the garklein in an orchestral context is, if you already have two lines for the tenor and the soprano, you can add a third one above for the garklein, for an added level of expressiveness. Sort of like what the piccolo does for the regular C flute.
The fingering is different from the rest of the recorder family. Thankfully, it comes with a fingering chart. You have to do more “half hole”-ing, especially for the lower notes. But I’m an amateur and I have little trouble adapting, I assume a professional would find it even easier.