I find this discussion on the Forbes website, which concludes that the Gretchen "Black" thing is nonsense, is convincing.
TL/DR Major points: Nowhere in the series is it mentioned that Gretchen's name is Black. Her married name is Schwartz (German for black) and her maiden name isn't mentioned.
Nowhere in the series is Gretchen shown to be the least bit concerned or a fan of Whitman, whereas it's quite clearly established for Gale.
The handwriting of the dedication and the handwriting of Gale's notebooks are very similar.
Also, this a quote from Vince Gilligan "Walt Whitman’s poetry was something that Gale Boetticher loved. It touched his heart and he wanted to share it with his new friend and mentor Walter White. And unfortunately the sharing of it and Walt keeping this book in hindsight proved to be a bit unwise."