hi,

gpat exam score is accepted   for admission in different top most pharmacy colleges in india, gpat score is valid for 3 years, also gpat score holders students get stipend of 12400 while pursuing regular m.pharma course. you should start preparing for gpat from 3rd year of pharmacy course, syllabus is based on b.pharma course, however only the subjects of gpat syllabus need to study , so you should take preparation studying your text books and previous years papers, another thing is since it is a mcq type exam you need to practice a lot of questions.  you may follow the below link to know about some comprehensive books for gpat,

https://pharmacy.careers360.com/articles/best-books-for-gpat