Dinner buffet ideas

Here are two great make ahead, gluten free, vegetarian dishes:
Spinach Artichoke Quinoa Casserole from Making Thyme for Health.
Quinoa Fiesta Enchilada Bake from Skinny Taste.

This is just my opinion, but does there have to be a meat dish? I think it would be easier on you to make one main everyone enjoys. Maybe a gorgeous, filling vegetarian entree such as a butternut squash tart (lots of good gluten free crust recipes now) and/or a salmon also?

I second the idea of a taco bar! (unless you're aiming for something a little fancier)

In that case I would do a tortilla española, aka a frittata with potatoes and onions. So good, so simple, nice looking, easy to serve. With the tortilla cut into wedges, add a green salad of kale or arugula, with some nuts and homemade vinaigrette, a side of roasted veggies (maybe rainbow carrots, honeynut squash or delicata squash) nice bread, and a protein. The protein could be a nice roast chicken or a steak, but put it later in the buffet, so that people take less and the tortilla can be the star. If you're feeling super ambitious an olive or roasted red pepper tapenade would do nicely alongside the tortilla.

You don't have to reinvent the wheel. There are many threads here for serving a group with many restrictions and yet making a nice party where people feel they are actually at the same table.
Most involve some kind of spread where the guests assemble their food from a choices of carb, protein, sides....all set out in different bowls, so no cross-contamination.
One idea which may work for your group is corn tacos or a rice spread, with various fillings (meat, fish, bean), sauces and salads. Dessert can be a fruit-something with cookies on the side.
Have a look at this selection of questions and answers here for more ideas:
https://food52.com/hotline/search?q=dinner+with+restrictions
I hope it's lovely and that you'll come back and tell us both what you served and how well it worked...;)

I would do eggplant parm in lieu of lasagna or make a lasagna with zucchini, which would take care of the vegetarian, the fish eater, and the gluten free :)

Instead of tenderloin (which is pricey), how about marinated flank steak? Or roast beef using eye of round (there's an easy recipe from America's Test Kitchen)?