IIIT Guwahati is an institute of national importance. As per the Indian Institute of Information Technology Public-Private Partnership Act 2017 by the Govt. of India it is authorized to award its own degree. The same goes for all the other 14 IIITs that were set up on a PPP basis between 2013-2016.
All 15 of these institutes are regarded as institutes of national importance and can give their own degrees. As these are autonomous statuary institutes and are not within the purview of UGC or AICTE. The degrees awarded by these IIITs, including IIIT Guwahati has a similar status to that of IIT and NIT.
To know more about the act, you can look up the Gazette of India dated 09/08/2017.