GTS is just when you trade pokemon in the world i think you did it before correct? GTS Negotiations is a live trade. It looks for somebody just like in random matchup. When someon e is found you both choose 3 pokes. Then you do a live trade!
Read the first post, it should help.
gts is when you search for a basic pokemon while gts negotiations let's you see the other person's pokemon and compromise what to trade