I must say this one thing as thirsfeld has already said. Bread is not a binder, egg is. But if done properly with the correct amount of fat and moisture you shouldn't need a binder because meatloaf is a force meat just like sausage therefore it is an emulsion. A properly mixed emulsion should not need a binder. But to play it safe you could add egg. Again bread is not a binder. Any thoughts pierino?

You could use rice flour, quinoa flour, or gluten-free cornmeal? Not all cornmeal is gluten-free. You just need something to absorb any excess moisture and bind - not so much for texture. Cornmeal might be delicious!

Lots of info for you here but I just wanted to say the only time I use breadcrumbs when making meatloaf is when a get the mixture too wet. I like to add onion by liquifying in the food processor raw and adding it that way. So sometimes when eyeballing wet ingredients I make it a litte too wet to hold a form. ( I don't you use a measured recipe just an ingredient list)

See my previous answer--rice krispies!! (Although I think of Panko as something I use for added crunch...why would a meatloaf recipe call for Panko?)

Oatmeal works great but I consider it in the grain family, cooked millet adds a nutty touch, and you can make it with no bread or grain fillers at all, just meat. The bread is for stretching small amounts of meat to feed more. It is not a binder so you can go without easily.

I've successfully used oatmeal in meatloaf recipes.

Pesach crumbs!! They work beautifully for a substitute.

You could grate a potato or use some rice instead. Or just omit entirely.