What does the Gita say about anger?

ध्यायतो विषयान्पुंसः सङ्गस्तेषूपजायते ।
सङ्गात्संजायते कामः कामात्क्रोधोऽभिजायते ॥

dhyāyato viṣayān puṃsaḥ saṅgas teṣūpajāyate
saṅgāt saṃjāyate kāmaḥ kāmāt krodho 'bhijāyate

The man who ponders over objects of sense forms an attachment to them. From that attachment is produced desire, and from desire anger is produced.

क्रोधाद्भवति संमोहः संमोहात्स्मृतिविभ्रमः ।
स्मृतिभ्रंशाद्बुद्धिनाशो बुद्धिनाशात्प्रणश्यति ॥

krodhād bhavati saṃmohaḥ saṃmohāt smṛti-vibhramaḥ
smṛti-bhraṃśād buddhi-nāśo buddhi-nāśāt praṇaśyati

From anger results want of discrimination, from want of discrimination confusion of the memory, from confusion of the memory loss of reason, and in consequence of loss of reason he is utterly ruined.

त्रिविधं नरकस्येदं द्वारं नाशनमात्मनः ।
कामः क्रोधस्तथा लोभस्तस्मादेतत्त्रयं त्यजेत् ॥

tri-vidhaṃ narakasyedaṃ dvāraṃ nāśanam ātmanaḥ
kāmaḥ krodhas tathā lobhas tasmād etat trayaṃ tyajet

This is the threefold gate to hell, destructive to the self: lust, anger, and greed. Therefore, one should abandon these three.

शक्नोतीहैव यः सोढुं प्राक्शरीरविमोक्षणात् ।
कामक्रोधोद्भवं वेगं स युक्तः स सुखी नरः ॥

śaknotīhaiva yaḥ soḍhuṃ prāk śarīra-vimokṣaṇāt
kāma-krodhodbhavaṃ vegaṃ sa yuktaḥ sa sukhī naraḥ

One who even in this world, before release from the body, is able to bear the agitations produced from desire and wrath is a devoted person, a happy person.