Winter wheat or Field or Cereal or Winter Rye are two choices. For me the Field or Winter Rye are the most easily available. What you want is something that will grow and hold that growth all winter to stabilize and hold your soil. Something like Oats, that winter kill, would not bne a good cover crop.

"...but that do not contain gluten (wheat and rye are out). "

Is there an allergy concern? I'm not sure, but doubt if wheat or rye covers would impart gluten to your soil or compost, were you to cut the above ground plant off for green material.

Especially doubtful if you didn't let it produce heads as most (if not all) of the gluten is in the berry.

Clover or other legumes should work.
I'm with alphonse on the gluten. Mow or turn under before it seeds. I like annual rye for its deep root system.

Maybe some of these will work,
" In the Fall, sow the following cover crops to turn into the soil in Spring: Alfalfa, Austrian Field Pea, White Clover, Alsike Clover, Crimson Clover, Red Clover, Purple Vetch, Hairy Vetch, Woolly Vetch, Common Vetch, Fava Beans, Wheat, Oats, Cereal Rye, Winter Rape, and Lupines.
?

Here is a link that might be useful: good information about fall and winter crops here

Thank you for some more ideas. My daughter has celiac disease and cannot have gluten. There is thought that oats can become contaminated with gluten by growing in the same area. My hope was next year to begin growing hulless oats as an experiment for us and I just thought the easiest thing would be to avoid any gluten in the garden all together as a preventative measure. I have enough headaches already.

Celiacs should avoid breathing dust from gluten-containing grains such as wheat and rye. The amino acid that creates a reaction will not pass to other crops through the soil, but the gardener has to consider what they are breathing in the garden.

Oats is more often contaminated by being mixed with other grains either in harvesting or in processing. Again, the contamination does not come from the soil itself.

Oh, I forgot about the cover crop. Crimson clover (Trifolium incarnatum) is a good choice since it is an annual, but avoid white or red clovers which can become perennial weeds.

Here is a link that might be useful: Crimson clover cover crop

I agree with the Crimson Clover. I grew it last year and I'm growing it again this year! I had excellent results!

My favorite cover crops are barley or oats mixed with fava or vetch. Rye will not die back in zone 6 but barley and oats will. You cannot kill rye next year until it blooms which is around May in your area. Vetch will also survive so I would only use it in beds you want to plant late next spring.

Here is a link that might be useful: Crop Rotation

Keeping in mind that the purpose of a winter cover crop is to help prevent soil erosion planting one that will die in cold weather may not be the best choice.
Winter, Field, or Cereal Rye can be cut down or tilled in before it heads out and produces seeds. I have tilled it in in early April up here before. There is no reason to wait until it "blooms".

You might consider small-seeded favas. Planting in September, they are very frost tolerant, grow well in the arid climate. Got mine from Seed of Change - they say it can be mixed with other stuff as well. In my garden, that would be weeds.

Here is a link that might be useful: link

OT, but kind of relevant!
Where I live, there can be pretty extreme spring/autumn gales.
I finally erected windbreak cloth last year and it makes a massive difference and doesn't look nearly as hideous as I thought it would. Wind's really, really hard on plants and it's well worth reducing it.
I would've voted for rye, favas, daikon and mustard, but rye's out. I though rye was ok? I know oats cross with wheat (woats?).
Basically the more diverse the cover crop, the better.

I will definitely look into the favas, I may even be able to grind them for floor. During the summer I hope to use oats and buckwheat for cover crops to add variety but will probably stick with clover and fava during the winter. It is just easier to stay away from potential problems with rye and wheat.

feijoas, I will be checking into windbreak cloth for next season. Eventually I will plant a windbreak but that will take years to mature. The cloth cannot look any worse than the milk jug cloches I use for new transplants. If I cannot find the fabric here, I may just wrap my western fence with plastic.

Windbreak needs to be around 50% permeable or the wind can hit the barrier and create massive turbulence on the other side.
Do you have a local ag/farm/feed store? They often stock that kind of thing. In case they do and you even get options, go for knitted over woven as it's heaps more durable. And black. Green really sticks out.
Favas rock. Fresh for anything, dried for falafel, frozen for dips and stews. I dunno about flour though, I imagine it would be pretty 'beany'.

Fava flour is often combined with many other gluten free flours. By itself it would be overwhelming but in combination, not as bad. I need to become familiar with our farm store.

So when the advice is "other legumes" for a cover crop, can I buy a cheap bag of dried beans (black beans, for example) and hand sow them?

Maybe that isn't cost effective in the long run, but if I already have some dried beans on hand, I might just use those on my relatively small plot.

I did that this past summer in my garlic bed - pulled the garlic in early july, planted a one lb sack of grocery store black-eyed peas - they grew like bandits until the frost, got about 2 feet high. I knocked them over, and shortly here I'll cover with a grass clipping leaf mulch for the winter.