what does gulliver and fellow sailors haveto signal for a pilot

We aren't apprised of what signal was used. We can infer lanterns were used to signal those ashore, who would than come and lead the ship safely to port.

We cast anchor within a league of the town, and made a signal for a pilot. Two of them came on board in less than half an hour, by whom we were guided between certain shoals and rocks, which are very dangerous in the passage, to a large basin, where a fleet may ride in safety within a cable's length of the town-wall.