First of all, you need to be clear about the syllabus of the subjects. Refer your GSEB textbooks for that, and also the GSEB website, to see if there is any change of topics from the previous year syllabus.

Once you have done this, thoroughly complete your GSEB textbooks. Solve each and every question from the book.

You do not need to use NCERT books for GSEB board exam, since the prescribed syllabus is given only in the GSEB books and sometimes questions are asked directly from it.

All the best!