According to the reviews online, the difference is that Tezpur University (TU) and Gauhati University (GU) is that the former is a central university while the latter is a state university.
There are many other differences between the two other than this.
Therefore TU and GU are very much different from each other when it comes to academics, teachings, curriculum, etc. So choose the one that suits your requirements, as it will shape your career ahead.