There are quite a few disciplines you can choose from when it comes to M.Tech programs at NIT Trichy. To put it straight, G1 is the first group with a few selected disciplines, G2 with another set, and so forth.
The only reason why the college has done this is to effectively manage the total seats available and divide them group-wise.
For example, a particular program having a 30 seat intake can possibly be distributed as G1 with 15 seats and G2 having 8 seats, and the remaining given to G3. This easily helps segregate and understand groups.
Keep in mind that the cutoff varies for each program. The only reason why these groups were created is to allot seats to each discipline.