So precious and pretty. I think it's still usable because, despite being a name from a fairy tale, it's reasonably normal sounding, not like Rumpelstiltkskin. I wouldn't be weirded out if I met a Gretel.
Reminds me of Hansel and Gretel. I think this name should be just as popular as Greta because they are virtually the same name, just with a more feminine spin. Cute.