As the name explains, a group discussion consists of a group of individuals having an intellectual discussion based on specific and usually trending topics related to tech. The key to being good in group discussion is improvisation and keeping up on your toes.
A key component of group discussions is current awareness. If you have been keeping up with all the innovations in the world of tech, then you are well prepared for group discussion.
The aim of the format is to think deep and provide innovative perspectives & arguments on the given topics. Dependent on the organizers, participants are given ~10 mins prior to a round to research and note down their arguments.
The purpose of the competition isn't to brute force your arguments on other participants (it's not a debate), instead you have to direct the discussion towards your arguments and explain it to both the judges & audience as to why your perspective on the issue is the superior one.
Dependent on the levels of moderation, one might also be required to direct other competitors to moderate the discussion (otherwise everything boils down to a fish market, which the judges hate and will cause the entire group to collectively lose points); which requires one to establish general rapport with other competitors.
The 1st round of most GDs require participants to record a video of themselves, presenting their arguments on a pre-determined motion. To prepare for this, speak a few times in front of a mirror; arrange your points in the following order : Introduction - Extrapolation - Conclusion; and record in a well-lit environment. The time limit for these videos ranges from 1-5 mins.
Group Discussions generally follow a Bracket Model or Round-by-Round Elimination Model.

The total no. of participants are divided into (usually) 2 groups (4 is rare) of ~6-8 members each, after which the top 2-3 participants from each group get promoted to the next round. This continues until the pre-determined no. of finalists are selected (~6).
All the participants are assimilated into one large cohort, assigned one topic and are instructed to begin discussions. As the rounds of the discussion progress, 1-2 participants get eliminated. post-this, the topic either remains the same or gets changed, and a similar process ensues. This continues till either a final group of ~6 members is left or a 1v1 discussion.
However, under the umbrella of group discussions, a few variations exist:
Group Discussions can last from anywhere between 10 mins to 1 hour per round.