I think I can give you some information regarding this as I have friends who graduated from NITIE.
Approximately a score of around 700 + in the GATE examination for the general category can get you a call for the interview rounds of NITIE. The total number of seats available for Civil Engineering students through GATE, PGDIE-144 & PGDPM-44, is 188.
However, it is a bit difficult to answer your second question, but if you fulfill the eligibility criteria and prepare well, you will have a fair and equal chance of getting through the interview rounds which are GD & PI. In NITIE Mumbai the maximum number of students are from the mechanical/ production branch and then Civil, CSE, and ECE branches.
NITIE does not disclose the shortlisting or selection criteria. You have to do well and have good scores to get through.