No, dogs entered in Gamekeeper classes only do not have to qualify for entry into a breed class.
If your dog was docked either in the UK or in a foreign country before 6 April 2007 then it may enter Crufts. If your dog was docked either in the UK or in a foreign country on or after 6 April 2007 it will not, under the law, be permitted to be shown at Crufts (where public are admitted on a payment of a fee).
Yes. However, it is important to know the times of each of the classes to which you have entered and be aware of any potential clashes.