What should I prepare to get an admission at NITIE Mumbai through GATE?

The first part of your question cannot necessarily be broken down. But, it can rather be said that you should prepare enough to secure a reasonable score in GATE to be shortlisted. 

• There are other things you should be aware of as well. For example, the GD topics usually conducted at the institute were not entirely technical and commonly included general question sets. 
• Talking about PI, you have to present your CV or application, as per requirement.
• Essentially, the admission procedure consists of two rounds of GD, followed by PI. 
• You should be well prepared since there is a screening procedure in either round. 
• The most vital thing is to make a good impression on the panelist.

However, it is essential that you put valid points when participating in one. Because this will determine whether or not you are the best fit among others.