What did Thomas tell the others at the Gathering? How do they react to what he says?

In Chapter 49, Thomas tells them the Maze was never meant to be solved. It's a test, a trial. He knows that each of them was taken when they were very young, stolen by the Creators. Each of them has above-average intelligence. After they were taken they were sent to special schools until the Maze was built. All their names are just nicknames: Alby for Albert Einstein, Newt for Isaac Newton, Thomas as in Thomas Edison.

There is some doubt about Thomas's story from Winston and Alby, but Thomas presses on. The Creators wished to see how the Gladers would react to what they call the Variables, and to a problem with no solution. The objective was to see if they could work together and form a community.

The Variables have included Teresa, the gray skies, the Walls not closing, and the stoppage of supply deliveries. All of these have been designed to test their will. The Creators ultimately want to be left with the strongest for their plan, but the Gladers have to earn it. The code is the key. "It was hidden in the wall movements of the Maze for a reason. I should know - I was there when the Creators did it."

The Gathering continues in Chapter 50. No one has a response to Thomas' revelation for a long time. He explains that there's a reason everyone who's been through the Changing remembers him. He and Teresa were part of the Maze Trials, but against their will. They helped design the Maze. He tells them about the telepathic link they share. Thomas explains that he's telling them all this now to gain their trust.

He lets this information settle in. Surprisingly, no one seems to be angry. Newt says aloud that its the Creators who put them there, not Teresa or Thomas. Minho wants to hear more about the escape plan. Thomas continues.

The shortened version of that answer is that Thomas tells them that in order to get out of the maze they must get the plans which were part of operation A.L.L.A.H and must sacrifice one of them in order to get out