I think the problem is that in the original recipe you would have browned the veggies in the roux, which develops flavor. Since you didn't do that, you might want to saute some onions and garlic until well browned and add that in. Other possibilities:
(1) Sauce may just need to reduce and become more concentrated
(2) May need more salt
(3) May need a little more acid (a bit of vinegar or lemon juice)
Yep, that'll be fine. For other purposes, it might not be your favorite - sometimes it can get a little more of the "slimy" coating that people don't like - but since you're cooking it in gumbo, it's all just going to get taken up into the stew (where it'll provide some thickening, as intended), so no worries.
A little more, if you're curious: okra contains mucilage, which is thick and slimy or gluey. If it's cooked quickly, it doesn't have time to release much. But when it's cooked long and slow, like in gumbo, plenty is released and dissolved in the soup, where it acts as a thickener. Frozen okra sometimes releases the mucilage more readily, presumably because the structure of the plant has been disrupted by the ice crystals formed during freezing. This might make it unappealing when eaten on its own, but if you're making stewed okra or gumbo, where it would all have come out anyway, it doesn't make any difference. (I suspect that good quality frozen okra, which has been flash-frozen, might fare better, but I don't have the experience to say for sure, and it won't make any difference to you!)
Best Answer
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.