The weight/payload penalties on an A32X or 737NG using GUI must be so severe that regular service with these types would surely be virtually impractical?
At the moment there is no real issue as the largest aircraft using GUI on a regular basis are FlyBe's 146's, a type which in itself is suited to short runways.
Although Guernsey does not attract as many holidaymakers as it did, say, 15 years ago, I think it is a bit shortsighted for the island's government to say that the airport's sole runway (at around 4800ft long) is adequate. It's as though they don't even want to try and get more people flowing through. Even just a modest extension of 1000ft would make a huge difference to the number of aircraft types able to comfortable use the facilities and encourage more companies to promote the island.
Like a fine wine, one gets better with age.