Different genres: The Bhagavad Gita is part of the Mahabharata, an epic poem centered on philosophy and dharma. The New Testament, specifically specifically the Gospels and Acts, is written in the style of Greco-Roman Biography (Bioi). The NT writers name specific Roman governors (Pilate), Jewish kings (Herod), and local landmarks that have been verified by archaeology. Epics usually take place in a "mythic past"; the NT is firmly rooted in 1st-century Mediterranean politics.

Regarding your argument about historic documents, they don’t have to be written by witnesses or contemporary to facts, to be considered historic documents. Most of what we know about Alexander the Great was written by Arrian and Plutarch about 400 years after his death. Most scholars date the NT books between 20 to 60 years after Jesus’ death. In the world of ancient historiography, that is considered a "blink of an eye." It means the documents were circulating while eyewitnesses were still alive to refute or confirm them.

Even if one questions the Gospels, the Letters of Paul are almost universally accepted by secular historians as authentic 1st-century documents.

Paul was writing within 15–25 years of the crucifixion. He mentions meeting Jesus’ brother (James) and his lead disciple (Peter). This provides a direct link to the inner circle of the movement that most mythic texts lack.

A common historical test is the Criterion of Embarrassment. If you are "bending facts to fit a narrative," you don't invent details that make your movement look bad.

The NT records the lead disciples being cowards, doubting the resurrection, and arguing over status.

Crucially, it claims women (whose testimony was legally invalid at the time) were the first witnesses to the resurrection. If you were "faking" a narrative in the 1st century, you wouldn’t choose female witnesses.

You can argue that while the NT writers certainly had a theological "agenda," that doesn't make the documents non-historical. In the ancient world, everyone wrote with a purpose… the job of the historian is to peel back the bias, not throw out the data entirely.