Of course, just because it's not a cure-all doesn't mean it has no value, so you can't say GTD is worthless.
Literal
Of-course all-purpose not [just because-からといって] value [subject-が] not-exist [it's not that-わけではない][because-から], GTD [topic-は] no-good [quotative-ということ][to-に][topic-は] not-become.
A layered logical argument. ~からといって…わけではない 'just because X doesn't mean Y' is a key grammar pattern for nuanced argumentation. 万能 'all-purpose, omnipotent' and ~ということにはならない 'it doesn't follow that' add formal precision. GTD (Getting Things Done) is referenced as a known productivity method.