I did the GAPAN test less than a month before I did stage 2. I do believe it helped.
They tests are not exactly the same, although they are very similar. I found CTC's tests to be easier then the GAPAN ones.
They say that familiarisation with the test can only improve your score by a max of 1 point but I found that it helped more by just giving me that extra bit of confidence to relax and perform as best I could.
Where you really get your money is the debrief. The people who do the GAPAN tests have been at the height of aviation. The Captain who did mine used to be a training Captain at BA. They can give you some very good advice and guidance which is invaluable.
I would highly recommend the GAPAN tests, £150 is a drop in the Ocean compared to £60,000!