hey buddy!!.. i gave my GD close to 2 years ago , so i may not be able to paint an exact picture for you but i will try..
At the start of the Assessment centre a representative from the HR will come and give a brief idea of the activities which will happen during the day, everyone will be given tag numbers and you will be judged or known according to that.. groups of 5 will be formed and ur group has to interact with another group of 5 during the activities..
There will be 5 topics , which keep on changing.. 2 are based on real life accidents or incidents and u have to debate.. 2 are " what if " kinda scenes.. u will be given a list of 10 to 12 things and u have to prioritize those things and come to a conclusion with your group.. 1 in which u will be given scenarios like " you are a commander and you see rubber pieces on the runway, take action... or you see a CB cloud in front of ur flight path, take action.. or there is a bomb threat on your frequency , take action....
these were the topics which were there at my time.. they change.. but you should be able to paint a picture with this..
cheers!! and best of luck