“You think the Geonim, who came immediately after the Gemorah, just invented it on their own for fun?”
There are numerous shitot of the Geonim that don’t appear in the Gemorah; some have become normative halachah even when they conflict with explicit decisions brought down in the Bavli. For some of these we do not know their source.
“You’re trying to overrule the Rema”
While I am a big fan of the Rema, it is a fact that many Ashkenazim do not follow the Rema on many matters. Do you keep an hour between meat and milk? Will you be donning tefillin a week from today? Do you recite Hallel in synagogue on seder night?