Hello Parth; 

For GPAT you have start preparing by yourself 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 other departments to be in like placement, their semester exams and so on. 

The best institute in Delhi for GPAT preparation, i will recommend RAHE institute. Provide good faculties with experience in the field. 

Hope it helps. 

Thank you.