If you create a new virus and it turns out unexpectedly to be more dangerous than the original one, it does not mean you are doing "gain of function" research

Gain of function research is research to create synthetically engineered version of viruses that are more dangerous than the ones that were used initially. While it can be argued that the intention was not to create weapons, the reality is that there is minimal to no practical value in this work unless the new viruses are in some way more dangerous. It may be that they are only more dangerous in that they are new and there is no immunity against them, but they are still more dangerous as a result. The only argument put forward for this work is that it enables new strategies to be developed 'in case' such viruses emerge outside of a lab setting. Therefore, again, there really is no possible motive for the work unless more dangerous viruses are going to be produced.

It just means that the result was you created a more transmissible virus.

No, it can also mean that the virus that you engineered has new traits which are more dangerous as well as transmissible.

Under your expansive definition, any research that modifies a virus is potentially gain-of-function research that would be illegal.

Not really, they were specifically trying to create viruses that pose a threat, specifically for that reason.

The author of the world and US bioweapons legislation, Professor Francis Boyle, himself has looked at all of the public documents on this and stated that this work constitutes Gain of function. How are you going to get someone more relevant to comment on this?

The letter is clear on the point that the behavior of the resulting virus in this case was not intended/expected.

Many statements have been made to that effect, yes, do you really think anyone currently working in a connected position is going to say anything different when their career and so much is on the line? Of course not.

the definition of such viruses is limited to human transmissible viruses.

As I understand it, the purpose of the work was partially to 'discover' human transmissible viruses - hence it was/is done in biosafety 3/4 labs.