How did Gulliver come to know of the intrigue plotted against him by the courtiers ? How did he ensure the secrecy of the courtier who informed him of the same ?

Gulliver was pretty oblivious to the treasonous dealings against him in the court politics of Lilliput. Gulliver had a friend, Redresal, of high ranking in the court of Lilliput. In the dead of night, just before Gulliver is plans to visit Blefuscu, he tells Gulliver of the plot against him. They plan to impeach Gulliver and have him put to death. I'm not sure I understand your last question correctly.