This ‘feature’ drives me completely bananas about gmail, though over the years I have gotten used to it and actually make use of it to intentionally fork conversations. I do sometimes wish I could merge them again later though which is not possible. (am I looking for the discourse admin functions wrench? maybe! 
)
I agree with @codinghorror that it would be ok with PMs but I wouldn’t want to see this on my site with normal topics. Sometimes topics get renamed. Life happens. Google is quirky sometimes, so what? It would be weird to the extreme for the subject line in the email to be different from the topic title. Sometimes the old topic is truly bad and needs to be changed for whatever reason so keeping it the same would be weird.
Maybe plugin territory, or an admin setting? E.g. “Strict mailing list mode. When mailing list mode selected, keep original topic title as subject in notifications even if topic title is changed.”
Or, duh: look for gmail addresses and send them different notifications? Wouldn’t work for google apps unless there’s another way to detect them.