Why is IIT Gandhinagar behaving more like a private college and less like an IIT?

I fail to understand what you mean by ‘behaving more like a private college’ and from where such a question stems from. If your comparison of IIT Gandhinagar with private colleges is to point out its features uncommon to other IITs then maybe yes, IIT Ganghinagar is certainly a bit different from the usual IIT standards. 

Consider the following points to understand this further:

• It is one of the fastest growing second generation IITs, that were established in 2008.
• The institute is well known for its highly flexible branch change policies, curriculum, cutting-edge research, and top-notch placement opportunities.
• The flexibility in the curriculum can be seen through the wide range of options of dual majors, minors, honors, course range, and short courses.
• It has ties with several foreign universities and thus provides great study-abroad opportunities to UG, PG, and Ph.D. students in the form of semesters overseas, research internships, summer programs, etc.
• Another distinctive feature includes harmony between different disciplines with 7 Interdisciplinary Research Centres. All the departments have common research facilities and there isn't any kind of monopoly.  
• The institute highly focuses on Project-oriented-learning that gives the students a practical approach to theoretical concepts.
• IIT Gandhinagar has some highly renowned faculty and the majority of the professors are PhD holders from top foreign institutes like Cambridge, Harvard, MIT, etc.
• Another unique initiative is the Non-degree Program wherein students from non-IIT institutes take part-time/full-time courses and get to spend an entire semester at IIT Gandhinagar. 

The list is not exhaustive and IIT Gandhinagar indeed is a unique IIT that has made its mark alone without the help of the IIT tag. It focuses on both the professional as well as personal development of its students, faculty, and staff. If you call such fast-paced growth ‘behaving like private colleges’ then it is what it is.