If you are going to be providing gluten-containing foods and gluten free foods, you will need to be sure that the area you prepare the gluten free foods is completely clear of gluten before you start and that the gluten free foods are kept completely separate once prepared and during transport so that there is no cross contamination.
I would also make sure that the gluten free options look completely different from the gluten options, so I wouldn't, for example, do sandwiches for both, I would make the gluten free option with gluten free wraps, say, so there definitely cannot be any mix up once prepared.
Make sure any cold meat is gluten free (e.g. some hams are breaded, some marinaded prepackaged chicken will contain gluten in the marinade).
Potential sandwich fillings - chicken breast with mayonnaise, cheese and tomato, ham and salad.
Crisps - check the labels e.g. some flavoured crisps will contain gluten.
Looking at packages - you are looking for any grain containing gluten being highlighted and not just the word "gluten" e.g. some crisps will contain barley so barley will be highlighted in the ingredients.
Or if you're really stuck, M&S, Boots and Tesco do gluten free pre-packaged sandwiches, M&S also does a gluten free pasta salad, could you go past one of them on your way to the course tomorrow morning - they do sell out quickly though as they don't stock big numbers, so it is a bit of a gamble.