You could do something like this

 mean = Reap[Sow @@@ Flatten[cogAK6, 1];, _, {Mean[#2], #} &][[2]];

This will be a lot faster than your approach because by using `Sow` and `Reap` this code only iterates through the list of data once. In your code, you reiterate through all elements of the data list for every value of `gazeNo` (so 3000 times instead of only once).