Being gifted doesn't excuse a child from being respectful to someone (who may be volunteering his time?).
I agree with Val on this.
I honestly don't have enough of a read on the situation (from simply reading posts here) to know what's really up - but if you haven't already done this, I'd meet (without your ds) with the adult that's sending the reports home and with the supervising adult at this activity. Ask for them to describe what's going on - listen to what they both have to say. If only one talks, ask the other if that's what they've seen.
There are all sorts of angles could be happening here. For instance, some adults are simply more mellow about overlooking back-talk (you stated above that he does talk back at home sometimes and you ignore it) .That's a totally legitimate way of dealing with it, but it doesn't make the behavior acceptable. It's possible that your ds has acted this way with other adults and he's now getting called on it simply because he's run into a supervising adult with a low frustration level for it.
OTOH, it's also possible that your ds is acting entirely different around this particular adult than he does around other adults - and if he is, there may be a reason for it - maybe the adult is the problem. If that's the case, you might seriously need to consider pulling him from the activity if there isn't a way to avoid the adult. I may sound too cautious about that, but I have known kids who've been hurt (physically and emotionally) by adults who took advantage of power in a situation like this. I am not saying that's going on at all - just saying that if you're seeing atypical behavior in one situation with one adult - it could be the adult that's the issue.
Best wishes,
polarbear
