GG Main Chamber has been laser surveyed but this didn't capture most of the great rift high up between the Rathole shaft, lateral Shaft and Main Shaft (due to droplets in the showering water disrupting the laser beam). If you've never visited this great rift you might not appreciate its size. If comparing GG with Titan (which is a shaft with a large top half, above the Event Horizon) you need to include the GG great rift to allow proper comparison. There is an article in a Descent magazine from a few years ago which points out that the volume of the great rift considerably adds to the volume achieved by the GG laser survey.
I' think Titan has been laser surveyed - Moose will probably be able to help if he sees this. But don't forget there's another 30 m depth of boulder filled shaft below the floor of Titan, leading down almost to Far Sump level. Then again, some geophysics done in GG tells us that the true floor of the Main Chamber is up to 30 m below where you land after abseiling into it. As GG has a much large floor area than Titan, if you include the buried part of GG it's probably bigger than any other known karst void in the country. If you don't include the buried parts of GG, Titan - and other possible contenders, then it's a much closer run thing.
What no-one disputes is that Titan is by far the deepest natural pitch in the country. GG isn't even the deepest in the Dales - Vulcan Pot (in Nick Pot) is deeper than GG.