Hello Aspirant; 

For GPAT,  you have to start very early from the last semester of 2nd Year  .And the best time to take coaching is end of 3rd year and starting of 4th year. As in 4th year,  students have lot more to do other than that. Like : Placement regarding and Semester exams and all. 

The best institute in Delhi for GPAT preparation is RAHE INSTITUTE. The faculties are quite supportive and experienced  too. 

Hope it helps. 

Thank you.