Warren has a cool animation showing the addition and redemption of pallets of gold bars out of GLD's vault, done in the style of the old PC disk defrag programs, at the screwtapefiles blog.
A few comments on the 5 minute animation (see screenshot below):
During the redemptions in 2013, most of the bars are taken from the bottom, that is, the recently added stuff. Makes sense, this stuff is easier to access.
But note much of the redemptions are from all over the place. Some of that is explained by them "hunting" for 9999 bars as Warren discussed in emails. He will have a follow up post taking this animation analysis into more detail showing this.
In the pic below the area just above the empty bottom area is really stubborn, something about those bars they don't redeem from, even though they are more recently added than the bars in the first half of the pic above that section.
Snapshot of GLD defrag youtube: