Tomato based gravy for brisket is too salty.
If you don't want to alter the overall flavor profile, then the only option is to make another portion of the same preparation without the salt and combine the two batches together.
Best of luck.
I would add another large can of unsalted whole tomatoes, crushed.
And/or some plain roasted or boiled root veg (potato, carrot, parsnip, turnip) when you reheat the brisket and gravy.