As FillyOfDelphi has said, DeGaulle was IIRC just a colonel; for this reason alone it would surely be someone else who is either boss or figurehead of a Fascist coup in France. There's no reason for DeGaulle to be involved as anything but a loyally serving French officer.
I am not even sure he would support a fascist coup. Sure, he was pretty right wing, a definite foe of Communists, someone who would happily see the whole ragtag social democratic left go into eclipse and become irrelevant. But I don't think he personally crossed the line to fascist himself; he was a right wing liberal I believe.
I think the best the actual fascists could hope for from him is that he stays loyal and quietly carries out his orders. I think it is actually possible he would join resistance against a truly fascist takeover. Discounting that as a long shot, he is just out of the picture politically, below the fray.
After all, at the end of the Fourth Republic, the right wing officers named putting him in charge as one item they were blackmailing France for, and it seemed to many that France had indeed fallen to fascism then. But only in the hyperbolic sense that someone like me might call Reagan or one of the Bushes that; I have to admit that technically they fell far short of a truly fascist takeover. I don't apologize for the hyperbole because I still fear that's where that trend in America takes us toward, and that every iteration of that wing in power does put us in more danger--but the line has not actually been crossed yet. Nor did de Gaulle impose a police state on France; rather he used the centralized emergency power the rightists connived to hand him to shut them down, take away one of the prizes they wanted to fight for (retention of Algeria) by settling with the Algerian independence movement, and any right wing atmosphere in France after that was because French majorities supported such politics, in the face of a strongly competitive and freely acting set of leftist parties free to register their dissent and propose other policy. That's not fascism! De Gaulle IIRC said it himself, that if anyone thought he wanted to be a dictator the more fools they, that he hadn't fought for French freedom to set himself on some sort of throne.
As I say, I have not made any close character studies of the man; God knows he was on the other side of many issues from me. But on some very important issues, he made what looks, from my distance anyway, to me like the right calls. Perhaps a de Gaulle biographical expert can show he was dissembling when he denied dictatorial ambitions, that perhaps he was just, on the cusp of the end of the 50s and start of the 60s, trimming his sails for a global reality dominated by the USA which would not tolerate a super-Franco in France? But hey, the USA certainly backed the actual Franco in Spain, and Salazar's dictatorship in Portugal--why not an authoritarian France, particularly a strongly anti-Communist one? It is possible that the French rightists were not the only authoritarians in the West de Gaulle's actions saved France from! At least as likely if not more so that that de Gaulle was fundamentally more inclined to support dictatorship than democracy anyway. As a liberal democrat, he backed what I think of as the wrong side, but he certainly seems to me to have been committed to preserving liberal democracy as such in France. (Maybe not so much in other nations and colonies-but anyway, for him I believe failure to preserve general democracy would feel like a great diminishment of French glory).
{edit--I removed a remark about him preferring monarchy to other forms of dictatorship--for the more I think about it, the more sure I am he was above all a republican. Maybe not "liberal" at all except in the formal sense parliamentary democracy goes with that term in the 19th century sense--but completely republican. Observe how he used referenda to validate his more radical moves, and when one failed, he promptly resigned. My impression is that to him, the glorious identity of France was bound up in the democratic legacy of 1789 and he would not fight for any cause exclusive of that. So maybe a parliamentary constitutional monarchy, but only one that put the real power in the assembly, and he would prefer a kingless republic far more. None of this contradicts his rightist lean by any means after all.}