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.)
I think it also bears pointing out that, in the US at least, we largely failed to detect that the virus was spreading inside the country for some time - even after knowing said virus existed and with plenty of warning, so you have to wonder in retrospect how effective some of these so-called surveillance systems even are. It's easy to not find anything amiss when you aren't looking.