Get lots of sleep the night before!
The aptitude isn‘t that bad really, just don‘t get stuck on the hard questions. You will not finish every section, so answer the questions you can answer easily, and carry-on. If, by some miracle, you have time, go back and try to asnwer the harder ones.
There is a spatial aptitude portion, if I recall, and it can be helpful to look up some practice questions in a GATB Practice book.
For the physical, mine was a piece of piss, and my grandmother could have done it.
That said, get rest, pace yourself, and make sure you stretch out beforehand. Stretch, don‘t jerk, your muscles during warm up.
You will not be running/stair-climbing very hard, just at a steady pace for a boring period. The push-ups aren‘t so bad, I was able to do 19. The sit-ups will burn your stomach, and the hand-grip thingy is a silly.... no matter how hard you grip, it won‘t seem like it moved. Just grip as hard as you can. You get 2 tries, I believe, per hand, and they take your best score.
That‘s all I can remember... good luck!
Not scoring well in any particular test doesn‘t automatically get you punted, I don‘t think, it just means you might not be suited to certain trades.