That is a tricky one Trailrunner. It is okay to not eat the cake. It is okay to be grateful for the thought but decline the offer because you "have never really eaten cake or desserts". Just because it is gluten free it doesn't mean that other cake eaters can't enjoy it - it's not like it is going to go to waste. Unless it is a truly crappy gluten free cake, in which case why on earth should anyone including you be obliged to eat it!
It is TOTALLY okay to say "that was very kind and thoughtful of you and I am sure many guests will enjoy it, but I don't feel like cake right now, thank you. " Big smiles. Big cheesy you would such a **** to get angry at me right now because I am being so polite no matter how you try to spin it smiles. Big I am not swallowing one bite of your guilt trip smiles.
We don't celebrate thanksgiving in my culture and I am so sad about that because it sounds like so much love and fun.