If you're referring to the provision that "The United States shall guarantee to each state a republican form of government", then first of all, that clause does not vest that obligation in the judiciary specifically, and secondly, the form of government is not what's at issue here. Gun control, however inexcusable it is, does not of itself alter a state's form of government any.
That clause was not put into the Constitution so that federal courts could endlessly second-guess state laws based on purely vague criteria. The people never would have ratified the Constitution if that had been the case.
Thomas may not agree and I think his remarks suggest that he has the incorporation of the Second Amendment in mind as a guarantee of a personal RKBA.
If a government can violate unalienable rights, hasn't it become the tyrannical government that the Founders said the people had the right to overthrow?
What's stopping the formation of the Islamic Republic of New Jersey, for example, if the BOR are not applicable to the States?
quibbling bumpkin