Frankly, I think it's horrible for various reasons:
1. To me
Gretchen isn't a name in its own right but a nickname for
Margarete or similar names. Furthermore the -chen ending means it's a diminutive form so it sounds like a nickname for a small child.
2. I don't like the
Faust association. I think the character
Gretchen is one of the most obnoxious characters in a theater play.
3. The English pronounciation just sounds weird and wrong to my ears.
4.
Gretchen (and
Margarete as a full name) is completely outdated here in Germany. It would seem strange to see it on a little girl.