Spell the notes of a French sixth chord in the key of B flat minor. The chord must be spelled using the correct voicing.

G-flat, B-Flat, C-Natural, E-Natural

The all augmented sixths reqiure the MINOR 6th scale degree which is always G-flat (as opposed to the flat-sixth scale degree. If so, a french sixth in B-flat minor would be different than a french sixth in B-flat major, which is just ludicrous!). A G-double flat against E natural produces a doubly augmented sixth (enharmonic with a major seventh), not an augmented sixth. The note of resolution of that G-double flat, the dominant of B-flat which is F would be its enhamonic equivalent, thus destroying the characteristic resolution associated with augmented sixth chords.

https://en.wikipedia.org/wiki/Augmented_sixth_chord

http://musictheoryblog.blogspot.com/2008/02/augmented-sixth-chords.html

You are correct

No mistake was made here. The sixth degree of the scale is already flattened by the minor mode, and need not be flattened again to produce an augmented sixth with the raised 4th degree of the scale above the bass.