Both use mainly the same things, both are started basically the same way.
Roux is flour and fat/oil cooked to what ever color wanted with other liquids added for zest.
Gravy is the same, but water or other liquids are added for bulk/volume.
Once a week I make a mess of gravy, usually with hamburger or other diced meats. Mostly white but other times brown, depending on mood. Then I use it in every meal or most of them anyway.
Tho sometimes fat/flour and then milk is added along with ether dried beef and or hamburger for ether SOS or chicken fried steak, with a bunch of pepper corns ground up for a creamy white gravy.
I love a good gravy on dam near anything or everything for my meals.
Neat thing abt gravy it can be scratch made from nothing more than fat/oil and flour, (kinda bland) but then the zing can be added before or after, but before is best. ( I like to cook meat/dried or not with spices right after the flour is added but before the liquid is added but spices are added before the flour is added) so the meat is flour coated and cooked along with the rest of the flour.
For some making a good roux is a challenge and the first time you will burn it, I remember my first try with others at a party, that we had not a clue on what we were doing, way before the internet and web.
We were heating butter and flour highly intoxicated and waiting for magic, and we burned it (roux made by committee is a disaster in the making) Major house smoke out by the time we noticed it.
Lmao, all that got done was the roux was burned and the house smoked up. The next day was a sucksess, because a coonass neighbor was asked to help.
But a good roux is great for dam near anything as a thickening agent for any thing that is watery and can serve as a great sauce base.
Roux and gravy basically the same, but roux is a gravy not completed at the time.
Ether way good eating..
Just a small note about gravy and roux,
Tho depending roux is always something handy in the fridge to add some sparkle to most meals.