An action that is prescribed, free from attachment, and done without liking or disliking, by one who does not seek its fruit, is called sattvic.
Explanation
Krishna is describing three types of action according to the three qualities (guṇas) of nature. Here he defines the sattvic (clear, balanced) kind: duty done without attachment (saṅga), without being driven by liking or dislike (rāga-dveṣa), and without craving a reward. The word niyata can mean a duty prescribed by scripture or one's own station in life; commentators lean on one or the other, but both fit the verse.