Other granthas are also there like the Vedas, The Ramayana, the Mahabharata, Manusmruti and many others. Why those are not celebrated for their jayanti, but only Gita?
We don't know the birthdays of any scriptures. Probably Gita's birthday can be tracked with certain planetary positions of the Kurukshetra war.
Gita Jayanti is birthday of Bhagvad Gita, the sacred text of Hindus. It's celebrated on the Shukla Ekadashi, 11th day of waxing moon of Margashirsha month of the Hindu calendar. It is believed the "Bhagavad Gita" was revealed to Arjuna by Krishna Himself in the battlefield of Kurukshetra (in present-day Haryana, India) [Wikipedia]
Creation of other scriptures are more likely to be expanded for several days. While Bhagavad Gita was a single day event, which is a primary requirement to have a birth'day'. It could have taken hardly 2 hours:
How much time did Sri Krishna take to preach Gita to Arjuna?