We can't be sure of the fact that only Gita has been fed to it; and as my reply showed, it is showing that the caste is fluid as the reply mentioned anyone can become a Brahmin.
EVERY SINGLE brahmin I have met was born into a brahmin household - except for some whites who are treated notionally like brahmins.
> There is no one true book for the dharmic faiths and over time the concept was rigidified by the people in power to keep their power.
The thing you see today is not what was intended. We have seen throughout in history when you give one group power, it tries to ensure that the power stays with it; this is what the so called upper castes basically did.
In modern times, why would you want to become another caste? In India, groups are dying to be called a backward or lower caste these days