I love that piece! I was just playing it the other day.
As for the Gb, I think it's just one of those notes that is awful on the flute, and it will still be that way even if your flute is in perfect shape. That's not to say that all flutes will be equal in regard to the bad notes, but I think some notes, especially Gb are just always notoriously evil.
I wish I could give you an easy answer.. although I think practicing harmonics might help (the overtones of the notes). There are some books that talk about harmonics, there might even be stuff online about it. If you get better at reaching that note, it might be easier to play it softly. And actually it's much easier to play softly when doing harmonics.
And just practicing your double-tonguing slow, slow, slow. You could also try mixing up the notes a bit, like taking that passage and starting with the Gb, for example:
Gb-Eb-Gb-Bb Gb-D-Gb-Bb and so on.
Or playing it backwards.. anything so that your fingers don't get locked up on playing the same thing over and over.
Hope that answers your question a bit. Maybe other people will have some ideas.