Sorry that your co-workers are giving you a big dish of guilt pie for the holidays.
You could take the easy way out and make Texas Caviar if you can tolerate legumes and night shades. (PM me if you don't find a good recipe in your budget.)
You could opt out, I had a co-worker who didn't believe in holidays, and she didn't bring anything or attend the holiday party. She stuck up for her beliefs despite peer pressure, so it might be worthwhile to think about why you don't want to participate and say "No thanks, due to health reasons I'm unable to attend." (Or you could say religious reasons, because you are in fact religiously avoiding gluten.)
The middle of the road would be to tell the person you've identified as the biggest gossip at work that you'd love to attend the party but don't want to have to confront everybody about what's in their dish and how they prepared it, because that would ruin the holiday spirit, Sadly, you'll be unable to attend, and hope nobody confronts you about it. Tell them you told them that because you know how sensitive they are to people's problems.
Good luck, stand up for your rights however you feel comfortable doing it, knowing you're doing it for your health.