The Gaṅgādhara inscription, dated to 523-524, is first inscriptional use of the term tantra according to Subrahmanyam, VBC, p. 7.
Bibliography:
Gandhasāra.
One of the few known Gandhaśāstras, it is devoted to scent, perfumery and cosmetics. See Mahes Raj Pant, On Reading Two Edited Texts on the Gandhaśāstra, Ādarśa, vol. 2 (2002), pp. 21-27, and references given there (an online PDF is available).
Vajravidāraṇāsādhana (Rdo rje rnam par 'joms pa zhes bya ba'i sgrub thabs). Tôh. no. 2913. Dergé Tanjur, vol. NU, folios 3121v.2-313r.5. Tr. by Jñānaśrī and Cog gru Rab zhi (Cog ru Rab zhi?).