On one hand, I've seen a rule from the Acharonim (I've also seen it in a Shach on Choshen Mishpat also but I can't remember where) that whatever the Gaonim say is "Divrei Kabalah" from the time of the Gemara (which implies that just as one can't argue on the Gemara one can't argue on them).
On the other hand, we find that Rishonim do argue on Gaonim (and don't argue on the Gemara).
Do other Rishonim mention this klal? Is there a machloken in the strength of the Gaonic tradition?