I've read Belsky's book. It seems like it has a bit of GTD in it, and there is one reference in the book to David Allen, but it's hard to know, for example, whether Belsky is really familiar with GTD. It's not like DA invented the separation between projects, actions and reference. The book is more wide-ranging than DA's GTD book, and is aimed at "creatives" (a term I don't particularly like, as it is has as many connotations as "makers").
The electronic system is an offshoot of Behance's paper products, which are attractive-looking. Levenger sells the paper designs in Circa format. The paper is clearly a relative of the Cornell note-taking design. I'm sure you could do GTD with either the paper or the electronic version. Or you could do something not as rigorous, which is probably where Behance's products are positioned and pitched in the marketplace.