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 Mistress Sassz'ine and Harjatan. 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. If the execute phase of a single target fight lasts at least a minute however, you can use Prolonged Power as your 2nd pot if the entire duration of the pot will be spent buffing up your execute phase.
As for food buffs, since stat weights could be all over the place now, your best food can be any of the 3: mastery, haste, and strength, Nightborne Delicacy Platter, Azshari Salad, and Lavish Suramar Feast respectively. Which food buff you pick is determined by which food buff/stat sims higher for your character. Rely on simming the actual food with your character rather than just going by what's your highest stat weight. Simming the actual food is much more accurate.
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. Especially since we now have Lightforged Augment Rune, which is literally the same thing but is reusable forever. It requires you to be exalted with Army of the Light in order to purchase it (you only have to buy once).