Here are some options.
Betty Crocker has a recipe using Progresso cream of mushroom (since it is a gluten free soup). However, it only has a 1 star rating, so proceed with caution here. You might need to make it ahead and start tweaking it.
However, there are alternatives that don't use canned soup. Here's one from Food.com. Sadly, it has a billion ingredients and this is the kind of recipe that makes my eyes glaze over ...
Here's another one from Food.com, a little better on the number of ingredients, but the recipe is not specifically a gluten free one and uses cracker crumbs for the topping.
Here's one from Celiac.com that uses Funyuns for the topping! Fun!
I personally think I'd combine the above 2 and make the 'from scratch" base and then cover it with the Funyuns if anyone at my house liked grean bean casserole.
And if you want to go in a different direction entirely try this recipe that I learned about from my friend over at Happy2BAHomemaker. BACON WRAPPED GREEN BEAN STACKS!
Personally, I like my green beans pretty plain. I don't make them into a casserole, I just cook them on top of the stove, in a pan, put in a tiny bit of onion and ton of bacon and cook them for a long time, until the bacon is done.