The word nirvāṇa literally means extinguishment. It implies the extinguishment of self. Bhagwat Geeta also mentions the word,
लभन्ते ब्रह्मनिर्वाणमृषय: क्षीणकल्मषा:|
छिन्नद्वैधा यतात्मान: सर्वभूतहिते रता: || 5.25||
कामक्रोधवियुक्तानां यतीनां यतचेतसाम् |
अभितो ब्रह्मनिर्वाणं वर्तते विदितात्मनाम् || 5.26||
But some Buddhists claim that this word is not present in Vedas but in BG that makes the fact of Bhagwat Geeta being post-Buddhist possible. I am looking for the usuage of the word in Vedas or Upanishads or scripture that predates Bhagwat Geeta.
P.S: In any case, absence of the word in Vedas doesn't imply BG to be post-Buddhist. Nirvana is a sanskrit word that means extinguishment & Brahman Nirvana simply means extinguishment in Brahman, etc. So, it's neither surprising to find the word in BG nor does it imply BG to be post-Buddhist.