Every other theory requires involving many more unsupported hypotheses.
This would be obvious to the majority of the HN population in most cases, but the disinformation in this case is apparently quite effective.
How do we know that the virus (or a precursor) wasn't already circulating in China, and was simply detected in Wuhan because of the fact that there are experts in coronaviruses who reside there?
(Also I'm not entirely sure your statement about the gain of function research is correct. A Google search doesn't seem to provide any evidence one way or another.)
So these experts didn't notice/detect a virus right under their noses, or they knew about its existence but could not tell anyone because of fear of the CCCP