From where should I start for gk?
Also if I enroll for SBI PO test Will it be valid till SBI PO

Hi Abhi,
The first best way to start is to read Newspapers. Make it a habit. Then slowly you can jump into yearbooks published by major publishers. Also, you need to keep a track on Weekly, Monthly current affairs released by well known online sources.
Cracku provides GK PDFs online. They're a part of your SBI PO Prep. You can enroll for SBI PO and Clerk Mocks from the link below.

https://cracku.in/online-bank-coaching