But one who knows the truth about the division of the gunas and of action, O mighty-armed one, sees that the gunas are operating upon the gunas, and so does not become attached.
Explanation
Krishna contrasts the person deluded by ego (in the previous verse) with the one who knows the truth. The gunas (guṇa) are the three strands of nature (sattva, rajas, tamas) that make up body, mind and senses, and also the objects they meet. The knower sees that action is simply these strands interacting, so there is no clinging (saṅga) to the idea "I am the doer." Commentators generally read "the gunas operating upon the gunas" as the senses, made of the gunas, engaging with objects that are also made of the gunas.