The new compilation is very impressive - I'm going to pick it up even though I have every track on other releases. I can't imagine a better single introduction to GG for a new fan - it spans their entire career and even includes the track "The Power and the Glory" which was never released on any other album (despite the fact that one of their albums bears the same name). Also, the new set eliminates the need to buy the last three albums, since it contains the best of all of them. Combine all that with likely better remastered sound and you have a winner.
However, if you are a completist, I'd still recommend all of the following:
Edge of Twilight (which nearly encompasses the LPs Gentle Giant, Octopus, Acquiring the Taste, Three Friends, and Power and the Glory) - contains several tracks not on the new set;
In a Glass House
Free Hand
Interview
Of course, truly fanatical GG completists like me will buy it all, in multiple formats, plus bootlegs and live recordings!
P.S. Where did you see it being released on 10/11? I want to pick up a copy!