Hi,
The gapf test is not that difficult if you're reasonably intelligent, a nice person, and able to work in a team. It's got a lot to do whether you're suited to becoming a pilot. There's little you can do if you aren't up to it.
The test is made up of 3 parts, an intelligence test, a motor skills test and then a personality test. In the intelligence and motor skills test, your ability to concentrate is also tested. The personality test is quite varied, but so some extent tests to see what you'll do if you're stressed/tired, whether you the type who likes to argue, which is dangerous in a cockpit. Remember there are maps in a plane, so there are going to be arguments :-)
Either way, there's not that much preparation that you can put into it, other than a few hours of ms flight sim and a good night's sleep the days before the gapf test.
Good luck!
K