Question: Can you be a JW ("Jehovah's Witness") who does not have the GB as their leader?
Answer: What do you mean by "Be a Jehovah's Witness" and who is being asked?
The Christian Congregation of Jehovah's Witnesses will make it know THEY do not accept people who claim to be Jehovah's Witnesses, but also refuse to accept the Governing Body as their leader. Such a person would be labeled an "Apostate" by Jehovah's Witnesses and any outsider asking about the status of such a person by a third party would deny said person is one of Jehovah's Witnesses. In fact Jehovah's Witnesses would be emphatic that such a person is not one of Jehovah's Witnesses.
You have to ask "who has the authority to decide who is and who is not one of Jehovah's Witnesses?" It is not like asking if a person is a medical doctor or not. Some board of medicine has been given authority to decide that and records or licenses can determine that.
I could say "I am Catholic, but I do not accept the Pope as my leader." The Catholic Church may have issue with you making the claim that you are Catholic.