The origin of the word ‘gumbo’ is okra. I don’t remember the whole story, but Gumbo IS okra soup. Jambalaya is a rice dish. Creole is a tomato based dish. I think the most important thing is that these dishes are rustic and can have almost ANYTHING in them from expensive cuts of protein to straight up vegetables. All are spicy but not necessarily ‘hot’.
when you added the trinity to the roux, you probably just overcooked the garlic, or could have gotten some of the garlic's germ in the mix (both leave bitter aftertastes). Simmering anything naturally mellows sharp flavors, as it gives the flavors time to marry and intermingle - that's the reason a lot of tomato based sauces are simmered slowly. trust me, ...
Being in hospitality industry, I am taught to follow the standard ratio 1 : 1 : 10 where 1 for Butter, 1 for Flour and 10 for Milk.
We should make a white Roux for white sauce, so don't over cook butter and flour mixture.