When I cook corned beef I boil it or roast it, I am not sure how yours was cooked  so I will give you some general gravy/sauce suggestions.

If I cooked my corned beef in the oven, I would wash it first and then I would put some veges under it. For example a potato, carrot, onion, garlic (whole cloves).  I would then cook it for the required time.

When It was cooked I would add water to the vegetables and much them up in a saucepan on the stove. I would use this liquid strained as a gravy. You could add pepper I would not add any stock or salt as corned beef is very salty anyway.

If I was boiling the corned beef I would boil it in water with an onion and a carrot some peppercorns and one clove.

If this was the case then I would make a white cheesy sauce to go with it. To do this I would melt 50 g of butter in a pan, I would then add 3 heaped tablespoons of plain flour. I would let the mixture cook for a few minutes being careful not to burn it. I would then add 400ml of warm milk ( I warm mine in the microwave first) slowly add the milk to the flour mix (Roux). I use a balloon whisk to make sure there are no lumps. Then  when the sauce is to the required thickness I would add 150g of tasty cheddar that has been grated. I would serve this with slices of the corned beef... Yummo!