Ok I'm going to be a party-pooper and dare to write a genuine, humour-free post.
I don't think a lot of people really prepare for it because of mid-years taking up all their time and concentration. The GAT may help you (it also very well may not), you never know what happens tommorow. Nobody should go in and take it as a joke (although try and have a bit of fun while you do it :p). If you're in there for three hours, you may as well make the best of that time and try your hardest, you won't lose anything by doing this, if anything, it is the opposite.
Technically you can 'prepare' be doing past GATs, but really, only if you seriously have/had nothing else to do and in your case (OP), you had mid years to worry about. Even then it's not something you should go all out for and study hardcore. So relax and just do your best tommorow
I'm sure you, and everyone else, is/are more than capable of writing essays and solving simple/general maths, science and humanities related questions.