“If there is no shitah in Mishnah and gemarah that say they are not chayov how can there be a rishon who says against a gemarah with no other gemarah supporting him?”
In some cases there are tanaitic sources outside of the mishnah that might disagree with the mishnah and gemara. An example is the case for women reading the megillah: There is a tosefta that can be interpreted as it being a problem for a woman to read the megillah for a man even though the mishnah and bavli definitively rule that a man can fulfill the mitzvah by hearing a woman read. Rishonim disagree and the machloket continues to this day, with the majority of Ashkenazic acharonim siding with the Rema, and Rabbi Ovadiah Yosef ignoring the Rema (as he should — he is Sefardic!) and ruling that l’chatchila a woman can read for a man.