Anyone can call someone dumb or a fag. That's easy. And rather meaningless. They've all degraded into generic words for things which are disliked. How far has language fallen? We used to accuse people of having mothers who smelled of elderberries, and now we just call them gay? That's just sad.
Be creative! Dig up an old word, an old phrase. Reference Shakespeare, or better, some awful contemporary of whom no one has heard.
Or if you're really feeling adventurous: don't insult them at all, and instead analyze what they are saying and see if it truly is gay and stupid.