You can't discriminate against someone for having a belief. You may be able to discriminate against someone for the expression of that belief. For example, Jehovah's Witnesses believe that blood transfusions are sinful. If a JW becomes a doctor, they cannot be discriminated against for holding that belief. However, if they refuse to give a life-saving blood transfusion to a patient, they can be disciplined for that failure notwithstanding the fact that it was driven by a protected belief.
This is therefore not a simple question of saying that GC beliefs are protected and therefore you don't have to use someone's preferred pronouns. Having said that, it would, in my view, be unreasonable to discipline a nurse for insisting on using a patient's first name when talking to them rather than their preferred pronouns. It would also, in my view, be unreasonable to discipline a nurse for failing to use the patient's preferred pronouns when talking to other medical professionals, particularly in a situation where the sex of the patient is important, as was the case here.