Bhagavad Gitā does talk about the Gunas - are there any other instances in the itihāsas there is the mention of three Gunas?
Yes, Itihasa other than Shrimad Bhagavat Gita talk about three gunas at many places. Below are some examples:
Mahabharata, Book 3: Vana Parva: Markandeya-Sanyasa Parva: SECTION CCXI: Sage Markandeya enquiring about:
Markandeya continued, "O Bharata, the fowler having expounded these abstruse points, the Brahmana with great attention again enquired of him about these subtle topics. The Brahmana said, 'Do thou truly describe to me, who now duly ask thee, the respective virtues of the qualities of sattwa, rajas, and tamas.' The fowler replied, 'Very well, I shall tell thee what thou hast asked. I shall describe separately their respective virtues, do thou listen. Of them tamas is characterised by illusion (spiritual), rajas incites (men to action), sattwa is of great grandeur, and on that account, it is said to be the greatest of them. He who is greatly under the influence of spiritual ignorance, who is foolish, senseless and given to dreaming, who is idle, unenergetic and swayed by anger and haughtiness, is said to be under the influence of tamas. And, O Brahmana rishi, that excellent man who is agreeable in speech, thoughtful, free from envy, industrious in action from an eager desire to reap its fruits, and of warm temperament, is said to be under the influence of rajas. And he who is resolute, patient, not subject to anger, free from malice, and is not skilful in action from want of a selfish desire to reap its fruits, wise and forbearing, is said to be under the influence of sattwa.
Mahabharata, Book 12: Santi Parva: SECTION CCCXV: Discussion between Yajnavalkya and Janka:
"'Yajnavalkya said, These three, O foremost of men, (viz., Sattwa, Rajas, and Tamas), are the attributes of Prakriti. These attach to all things of the universe and always inhere to them. The Unmanifest Purusha endued with the six Yoga attributes transforms himself by himself into hundreds and thousands and millions and millions of forms (by embracing these three attributes). Those that are conversant with the science of Adhyatma, say that unto the attribute of Sattwa is assigned a high, unto Rajas a middling, and unto Tamas, a low place in the universe.
These are only some examples. There are many more.