How to do the chat gui thing?

like sandbox wherein a server is talking how? also how to center guis?

Using the Chatted event, you can pretty much do anything with chat. Of course, you'd need to improvise with the code inside.

game.Players.PlayerAdded:connect(function(child) 
	child.Chatted:connect(function(msg)
	-- Modify here for your game. etc.
	-- Example below: (you would need a ScreenGui with TextLabel inside PlayerGui)
	local p = game.Players:GetPlayers()
		for i = 1, #p do
			p[i].PlayerGui.ScreenGui.TextLabel.Text = child.Name..":".. msg
		end
	end)
end)