Is it true that GNDU gives marks on the basis of the number of pages?

It really depends on the person who is evaluating your paper. But most times, if the paper is theoretical, it is true. For theoretical papers, try to write 3-4 pages of answers for 5 marks of questions. 

But some teachers do look for quality content rather than length. So, it’s better to write answers with good content but also try to make it a bit lengthy. That should earn you a good GPA.