Apple Watch series 6 gps + cellular from USA

The cellular will not, however if you pair it with your phone, you can use it as long as you have the paired phone with you. To use an Apple Watch with cellular and activate the cellular in India, you need to purchase a Watch in India.

No.

As fieryjack has said, that is not possible. You cannot purchase a device designed for a different region in the US. They do not stock or sell those products. Make your purchase in your country.