What is dharma according to the Gita?

श्रेयान्स्वधर्मो विगुणः परधर्मात्स्वनुष्ठितात् ।
स्वभावनियतं कर्म कुर्वन्नाप्नोति किल्बिषम् ॥

śreyān svadharmo viguṇaḥ paradharmāt svanuṣṭhitāt
svabhāvaniyataṃ karma kurvan nāpnoti kilbiṣam

One's own duty, though defective, is better than another's duty well performed. Performing the duty prescribed by nature, one does not incur sin.

स्वधर्ममपि चावेक्ष्य न विकम्पितुमर्हसि ।
धर्म्याद्धि युद्धाच्छ्रेयोऽन्यत्क्षत्रियस्य न विद्यते ॥

sva-dharmam api cāvekṣya na vikampitum arhasi
dharmyād dhi yuddhāc chreyo 'nyat kṣatriyasya na vidyate

Having regard to your own duty also, you ought not to falter, for there is nothing better for a Kṣatriya than a righteous battle.

यदा यदा हि धर्मस्य ग्लानिर्भवति भारत ।
अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम् ॥

yadā yadā hi dharmasya glānir bhavati bhārata
abhyutthānam adharmasya tadātmānaṃ sṛjāmy aham

Whensoever, O descendant of Bharata, piety languishes and impiety is in the ascendant, I create myself.

परित्राणाय साधूनां विनाशाय च दुष्कृताम् ।
धर्मसंस्थापनार्थाय संभवामि युगे युगे ॥

paritrāṇāya sādhūnāṃ vināśāya ca duṣkṛtām
dharma-saṃsthāpanārthāya saṃbhavāmi yuge yuge

I am born age after age, for the protection of the good, for the destruction of evil-doers, and the establishment of piety.