Maguy is a French diminutive (or a pet form) of Marguerite. See
here.
I know basic French but I'm not fluent in the language. Since I'm hearing impaired and I don't speak very well, I don't know how to pronounce the name. Maybe something like Maggie but with the accent on the second syllable? I don't know about the French version of Grace, though.
There are several French members here...I'm sure one (or more) of them knows.