Gems & Enchants
Your gems and enchants that are just stats will purely depend on which stats are more valued to your character based on the sims that you ran for your character. It is very likely that your best gems/enchants are either the haste or mastery gems/enchants.
For flasks, you will want to use Flask of the Countless Armies since that's really your only option.
For potions, you want to use Potion of the Old War, but you can use Potion of Prolonged Power as a cheaper alternative or if it's a consistent aoe fight like Skorpyron. Potion of the Old War ticks can be buffed by all of our buffs and has the potentially to really boost our dps if you get a few good procs during Battle Cry. Check out the video on this page to make sure you are using Old War optimally.
As for food buffs, you will either use the mastery or haste food buff, Nightborne Delicacy Platter or Azshari Salad respectively. Which food buff you pick is determined by which food buff/stat sims higher for your character. If someone puts out a Lavish Suramar Feast, you can use it since it's "free", but generally it will net you slightly lower dps to use the 500 strength food buff.
You can also use Defiled Augment Rune to boost your dps even further since you can use it in addition to all of the buffs mentioned before.