Mr. Gabb is fully aware that he is demanding that the Royal Sovereign stand up for the rights of her subjects and due her duty to protect them, even if her title what has become a gilded cage is imperiled. Parliament, the BBC, and Brussels are trying to destroy traditional Britain. There already is a Constitutional crisis, and there is neither consent of the governed nor a pretense of legitimate authority.
If true, then Mr. Gabb miscalculated badly. The U.K. is not America, and HM Queen Elizabeth II is not an elected President. What Mr. Gabb has done, is publicly insult the Sovereign.
Had he been astute, he would have described HM Queen Elizabeth II as a noble and good-hearted but too-trusting Queen, taken advantage of from below by scheming parliamentarians. She and her noble court have been imprisoned by the parliamentary pandemonium.
That's how one does it with respect to the Monarch. The way he did it, he burned his bridge. The only hope he has is the chance that no-one in the court (i.e., the ruling class) will find what he wrote.
I realize I've stiffened up, but what he did was not dissimilar to an American publicly calling a sitting President a traitor. Only, the successors to the throne are descendants of Queen Elizabeth. Mr. Gabb has zero chance of influencing Prince William once the Prince finds out that Mr. Gabb publicly insulted his grandmum.