Depending on the gathering, you may also be able to get away with saying, “Oh, not just yet, thanks,” for the first half, and “Oh, no more, thanks,” during the second half. Granted, that works more easily at a buffet than at a sit-down dinner.
Another strategy, recommended by Miss Manners, is to eat to satiety at home, so you can then say, perfectly honestly, that you’re not really hungry. If pressed, you could then take a small amount of the non-keto food, and then proceed to mess it up on your plate, so no one can tell you haven’t eaten any. If they give you more than a small portion, you can cover yourself by announcing, “Oh, that’s far too much; I’m not going to be able to eat all that.”
Miss Manners also says that if you refuse food politely (i.e., with a “No, thank you”), the burden of rudeness lies on anyone who refuses to take that for an answer. Although in some cultures, good manners requires food to be offered and refused twice, and it is the response to the third offering (“yes, please,” or “no, thank you”) that is definitive.