I'm not sure that bringing a gf chicken and dressing is really going to solve anything, Iris.
It seems like what you have here is either ignorance so profound it's basically willful, or an active desire to put SIL's wants over your husband's and daughter's needs.
If it's willful ignorance, your offering to bring a gf chicken and dressing won't fix anything. They still won't understand, and it's still possible for the salad and even the gf chicken (nevermind the ham) to get contaminated and become inedible.
If it's plain old selfishness, your bringing a gf chicken and dressing is likely going to read as capitulation or as an outright challenge. If the latter, things could get very unpleasant, and you won't have really demonstrated where your line is.
If you think it is simply ignorance, I would schedule a time to talk to them. Sit them down and explain to them in detail what Celiac disease is (and what it does to a person who has it), what foods contain gluten that you would not expect, and how to avoid contaminating gluten free foods. Be exhaustive, and use past meals as examples to illustrate what you are trying to explain. Don't be afraid to put them on the spot a little bit -- if they are still ignorant of what is necessary, it is because on some level, they don't care enough to educate themselves. They deserve to be put on the spot for that. They deserve to feel uncomfortable for the fact that two members of their family have been made to feel unwelcome at every meal they have hosted because of that. And tell them at some point in the conversation that if you do not feel they are able to provide a non-contaminated gluten free meal, your family will be eating at home and joining everyone else after dinner.
If you think it's selfishness, then honestly, you're going to need to be a lot more confrontational to change anything. Your choices, at that point, are to avoid confrontation by just eating at home, or to confront the situation head on (using strong but polite language) in advance so that you can hopefully reach resolution by the holidays. But be aware -- if selfishness is the motivation, just about anything you do outside of the status quo is likely to produce a strongly negative reaction. This is not your fault, and you should not allow anyone to make you feel guilty for it. Best case scenario, your standing up to them changes the status quo and you never have to be so confrontational again. Worst case scenario, you have brought an attitude that already existed and that they aren't willing to change into the open for everyone to see (which is uncomfortable for them) and you limit your time with this part of your family as a result. Even in the worst case scenario, there is no reason for your family to sacrifice your comfort for the sake of other people being able to stay blissfully ignorant. You are responsible for the comfort and well-being of your family, and the way people choose to respond to your politely but firmly standing up for your own family is not within your control, and thus not any concern of yours.