In plain words
The agent who is free from attachment and egoistic speech, endowed with firmness and enthusiasm, and unchanged by success or failure is called sāttvika.
Word for word
Free from attachment, not given to saying 'I', endowed with firmness and zeal, unchanged in success and failure, the agent is called sāttvika.
मुक्तसङ्गोऽनहंवादी धृत्युत्साहसमन्वितःसिद्ध्यसिद्ध्योर्निर्विकारः कर्ता सात्त्विक उच्यते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
इदानीं कर्तृभेदः उच्यते —
मुक्तसङ्गः मुक्तः परित्यक्तः सङ्गः येन सः मुक्तसङ्गः, अनहंवादी न अहंवदनशीलः, धृत्युत्साहसमन्वितः धृतिः धारणम् उत्साहः उद्यमः ताभ्यां समन्वितः संयुक्तः धृत्युत्साहसमन्वितः, सिद्ध्यसिद्ध्योः क्रियमाणस्य कर्मणः फलसिद्धौ असिद्धौ च सिद्ध्यसिद्ध्योः निर्विकारः, केवलं शास्त्रप्रमाणेन प्रयुक्तः न फलरागादिना यः सः निर्विकारः उच्यते । एवंभूतः कर्ता यः सः सात्त्विकः उच्यते ॥ २६ ॥
In plain words
Now the kinds of doer are taken up —
'Free of attachment' means: one who has released and abandoned all attachment. 'Not one who speaks of I' means: not given to asserting himself. 'Endowed with steadiness and enthusiasm' — steadiness means sustaining resolve, enthusiasm means active exertion, and 'endowed' means joined with both of these. 'In success and failure' refers to whether the action being performed achieves its fruit or not; and in either case, he remains 'unaffected' — moved solely by the authority of scripture, never by passion for the fruit or anything like it. That is what 'unaffected' means. An agent who is exactly this way is the one called sāttvika.
Close to the Sanskrit
Now the division of the agent is spoken of —
'Free of attachment' — 'released', 'abandoned' — attachment, by whom; he is 'free of attachment'; 'not one who speaks of I' — not having the habit of asserting himself; 'endowed with steadiness and enthusiasm' — steadiness is sustaining, enthusiasm is exertion, endowed, joined by those two, is 'endowed with steadiness and enthusiasm'; 'in success and failure' — of action being performed, in the accomplishment of the fruit and in the non-accomplishment, that is 'in success and failure' — 'unaffected', impelled solely by the authority of scripture, not by passion for fruit and so on — whoever is this, he is called 'unaffected'. Such an agent, who is thus, he is called sāttvika.