I think it depends what your weak areas are. You will have little time for reading whilst on your GTP and it will be very much a case of reading what you need to for your essays etc. I knew I would have to spend quite a bit of time working on my rusty maths, so spent the time doing that, as I aimed to pass all three tests asap to get them out of the way. I booked them as soon as my number came through and got them out of the way by the end of half term. This worked well as I had passed them before the amount of teaching started to step up too much. I would advise you to look at the tests now and see if you think you might need time to get you up to speed on any. When I took the ICT one it needed a lot of practising as the software was so different to the ones used generally. If you think you will be ok on the tests, relax and enjoy the calm before the storm and read up on a few things you know you will need to (like behaviour management etc).