How do I politely let a host know about my dietary requirements?

Just pull the host aside or if you know ahead of time, call the host and ask what they will be serving. Tell them that you don't mean to be rude but you only ask because you have an allergy to gluten based foods and will get sick. You should know what types of foods have gluten in it so you can make the choice on whether you can eat it or not but some spices/seasonings may have it in them too.