Gluten Free and Vrtually Meat Free

Beans, lentils, quinoa...

By replace and given what you said about portobellos, I'm guessing you want something with a meat-like texture? The best vegan thing for that is seitan, but since that's pure wheat gluten, that wouldn't be good for you. I would suggest well-pressed tofu. (Drain the water it comes in, give it a squeeze and a rinse, wrap it in clean kitchen towels, and then sit a big coffee-table book on top on it to distribute weight. On top of the book, put a brick or a gallon jug half-filled with water or a few cans of canned beans/veggies, anything heavy but not too heavy, on top of the book. Let it sit for a half-hour so, until the extra water is pressed out. You may need to change the towel a time or two.) Slice it up, and with appropriate spices (Tofu is tasteless itself, but it absorbs whatever tastes you surround it with, so rub it with poultry seasoning or marinate it in what you'd normally marinate meat in) and cooking methods (like grilling, if you have a grill, or stir-frying or even roasting), tofu can be very close to meat (chicken breast in particular) in taste/texture. Also, tempeh, which when crumbled and appropriately spiced mimics ground meats pretty well.

If you don't want to to the soy route, well-cooked lentils can also work well for this purpose. So can ground-up root vegetables like beets. (I've fooled carnivores with the "beet burgers" I make, especially because the beets give them a red, rare-meat color. They don't grill well, but they pan-fry nicely.

But really, if you're going to do this, I'd suggest moving away from thinking that you actually need something "meaty." You don't, really. Be creative, expand your horizons, and check out non-meatlike sources of protein. Beans, for instance, are pretty darn amazing when you explore all the kinds and all the things you can do with them. I have whole cookbooks devoted to solely to beans.