1. Gain of function research primarily uses samples collected from nature, and seeks to stimulate their evolution in as natural a way as possible to learn how viruses evolve in nature. If such viruses were to escape the lab, they would appear "natural"
2. It's not xenophobic for people from the US to suggest the possibility of a lab leak, because the US was itself funding gain of function research on novel coronaviruses in the Wuhan BSL4 lab
3. Lab leaks happen more often than most people realize[1]
[1]https://www.vox.com/future-perfect/2019/3/20/18260669/deadly...
There is, from my understanding, reasonable evidence to conclude the virus was not engineered from the perspective of "we took genes from one virus and moved them to this virus," but there's no evidence disproving the idea that it was the result of gain of function research.
My personal feeling is that these statements are true:
* The virus is unlikely to have been engineered (in the way I described above) and leaked.
* There is circumstantial evidence the virus was the result of gain of function research and it leaked.
* There is circumstantial evidence the virus was a natural research sample and it leaked.
* There is circumstantial evidence the virus was introduced by an animal/person who traveled to the wet market.
Some of these are more likely than others, and an individual's own calibration for what is likely or unlikely will probably come into play more than evidence in the short term and possibly long term as well. I can say the vast majority of us are not qualified to answer the question either way though.
It's also worth noting that even the leading experts can get these things wrong, as was the case with the Sverdlovsk lab leak.
Soviet authorities covered it up by blaming local meat markets, and leading US experts concurred with them, only to reverse their conclusion 6 years later.
https://en.m.wikipedia.org/wiki/Sverdlovsk_anthrax_leak#Acci...
Which will always look suspicious, whether it was actually a completely natural virus or not.
The Chinese authorities are not stupid. They know how bad it looks to not allow investigation. Which means they think the result of a free investigation would look even worse.
Consider for a moment the propaganda value of being able to say, "we visited their labs and witnessed first-hand their failure to {do some sanitization procedure} correctly! Oh how they clearly don't value human lives!"