For a referral to another medical specialist the GP will usually provide you a letter of reference to be given to the specialist, whom you in turn will call for an appointment. You may have to wait several weeks or longer for an appointment unless the matter is urgent. Some insurance companies can work with you to help to speed up your appointment.
Without a referral note from your GP, it is impossible to visit a specialist. Above that, without the referral note, your health insurance company most likely will not reimburse you.