The crustless pumpkin pie is just the usual pumpkin filling baked in a lightly greased pie pan or plate. No pastry or crust. Take a can of prepared pumpkin pureé or pumpkin pie filling and follow the recipe on the can. It still has the usual eggs, milk, sugar and seasonings added.

Frankly, I prefer to use a pastry shell. The "gluten-free" thing is just a diet fad without much scientific support or even weight control sense. Or if "no carbs" is the excuse, I feel much the same. Carbohydrates and vegetable proteins have been part of our diet for millennia. A few people have special sensitivities, yes, but the vast majority of us do not.