Once you have selected your gondolier and asked him to take you for a ride, use courteous language, and after the customary rites and small talk appreciation of the city, on food, on the latest regatta and whatever other argument takes your fancy to see if your interest is recirpocated and before coming to the point, it is sufficient to ask the following question:
Before I leave Venice, I would like to see the sites behind the island of San Giorgio. Would this be possible?
The indication of the place and the space behind this ancient Benedictine monastery, magnificently retouched by the architecture of Palladio, has a precise connotation and you will immediately understand from the gondlierís reply, his eventual availability. But as mentioned in the above paragraphs, it is most unlikely that he will make himself available before the end of the day.