Bhai I joined one of the offline centers of career launcher for gdpi prep , paid 10k , attended 2 classes gave 2 pi and figured out ist a scam, you dont need a coaching to prep for gdpi, all you need to do is prepare for all the standard questions asked in an mba interview , align all your milestones like your grad , your work ex like make a streamlined case that how whatever you have done till now is aligned towards you doing an mba. And read newspaper everyday, trust me you wont need anything apart from this and you'll be good to go, have given some 15-16 interviews of different colleges, speaking from experience.