I did.
I ordered it a few months ago and listened to it a couple of times. I then loaned it to a friend so I haven't heard it lately. I would need to hear it a couple more times to go into more specifics, but I thought I'd give you guys an initial report.
I believe I have heard all of the other completions (Cooke I, II, III, Wheeler, Barshai, S&M, Mazzetti I and II, Carpenter, missing any?).
I think the Gamzou presents the most important step forward since Cooke in providing new insight into Mahler's intentions. His choices seem much more Mahlerian than some of those by Mazzetti, Wheeler and others--there are relatively few moments that sound 'wrong' for Mahler.
He's clearly put a lot of time into each and every moment of the piece. There are lots of noticeable little differences. Many of them work well, many are neutral, some don't work as well.
What he does with the Finale is his greatest contribution. While Cooke seems to have "corrected" a lot of "wrong" notes. Gamzou gives us a much closer picture to the piece Mahler intended as far as dissonances and non-diatonic harmonies go. The lush string parts of the Finale sound nuanced and boundary-pushing. The movement sounds more like it belongs in a symphony with the first movement.
I have always felt that the second half of the Finale seems too pedestrian for Mahler. Indeed, this movement still reveals itself as a work in progress, but with the Gamzou completion a tension between diatonicism and atonality is now apparent. You get the sense that this movement was meant to present a yin/yang juxtaposition between the old and the new, rather than simply present a derivative tonal ending.
I think Gamzou's tempi need work, especially in the Scherzi. They feel a bit overfussed over, too much accelerando/ritardando, but the benefit here is that a number of new tempi ideas are presented, some of which do work quite well. I suspect that Gamzou or another conductor would be capable of better presenting this new score after performing it a few more times.
The tempo trouble and the quality of the recording (sounds like the peaks are somewhat clipped) keep this from being THE definitive source for M10, but it's in the top 5 recordings for me, and probably the only non-Cooke version I can accept without any major reservations.
I think it's an exciting step forward. The next completion should definitely look at the new ideas Gamzou has presented.