Can I use just flour instead of roux in my gumbo
I wouldn't recommend it.

Dark roux is actually a pretty weak thickener compared to a light roux.  As you get darker, it's more of a flavoring.  Flour on its own gives an obvious raw flour taste.

As an alternate method to make darker rouxes, search for 'oven roux', where you're working with a more easily controlled, even heat, as opposed to something that you have to keep stirring.

If you're still scared of roux even after that, I'd recommend a slurry of cornstarch or arrowroot over plain flour.