They do provide a special erasable marker, so don't worry about the pen. I'm not sure if you can chew gum, but you can't have the water bottle with you while answering questions. They will provide a locker before you start where you can store water and snacks for your breaks.

You will only be able to bring your locker key and ID into the testing room. Everything else has to be secured inside the provided locker.
You are not allowed anything around your wrists and your pockets must be empty each time you reenter.

Dana is right about the water bottle and I'm not sure about the gum either. My guess is they will probably not allow it as it might be distracting to others.

You will probably be given one pen. At my test centre they were nice and gave me two. They do tend to dry out so keep the lid on them when you are not using them.

Don't forget to make sure your ID name exactly matches the name provided.
Don't forget to eat a good breakfast.
Don't leave your seat without raising your hand.

Good luck!