Your husband is a very intelligent man. He knows that the GP is not a specialist. He also knows that she cannot make further referrals or investigations if he does not want them.
You have provided the GP with your observations, but you don't know what he has said to her. She could have, in all good will, said that she'll take further action, only to then check his notes and see that he has refused any further investigations. She cannot tell you this.
Even if the GMC decide there are grounds to investigate, they have no power to alter your husband's care. That has to come from him. Also they'll be very limited in what they can tell you, as anything concerning your husband's care will be confidential.
I'd also keep in mind that, once the GP is made aware of your complaint, your husband will become aware. He'll need to authorise access to his medical records. If you don't feel safe to tell your husband that you've contacted the GP, what will happen at that point?
I completely understand your concern and the strain this must be putting you under, but your problem is solely with your husband. He has the capacity and the capability to get the help he needs, but he refuses to do so. Nothing will change until he chooses to act, or in the unlikely situation that he is deemed to not have capacity.