This shloka is the turning point of the chapter. Krishna says: when a person sees clearly that the gunas alone are the doers of all action — and simultaneously knows the reality that lies beyond the gunas — that person attains My nature.
Every time we say "I did this" or "I achieved that," the truth according to the Gita is that the gunas did it. Sattva prompted the clarity, or rajas fuelled the ambition, or tamas caused the neglect. The soul itself is the witness, the drashta, sitting behind the screen and watching the play of the gunas unfold. The moment this becomes a lived realization rather than just an idea, the grip of the gunas begins to loosen.
And there is a second step: knowing what lies beyond the gunas. That "beyond" is the true Self, the atma, which is one with Krishna's own nature. Recognizing the gunas as the doers is half the journey. Knowing oneself as the witnessing consciousness beyond them is the destination.