The other thing about foods thickened with cornstarch is that they don't keep. Within a certain amount of time (don't remember how much, but less than 48 hours), they tend to separate and "weep".

Tapioca (not the pearl kind) and arrowroot are effective thickeners that are also, coincidentally, gluten-free.

Pan gravy -- the reduced but otherwise unthickened juices from the roasting pan -- is also nice, and safe for your celiac friends.

Me, I don't think it fair to invite people to dinner, require them to bring a dish, and then lecture them at length about dietary restrictions. The host is responsible for ensuring that every guest will eat well, and if that means farming out only the un-challenging parts of the menu, or providing both conventional and adapted versions of particular favourites, then so be it.

Speaking as one who does her best to avoid sugar ... !