Yes, tomatoes can help balance salty gravies due to their acidity and natural sweetness, which counteract the saltiness.
Tomatoes contain citric and malic acids, which provide a tartness that cuts through the overwhelming saltiness in a gravy. Additionally, the natural sugars present in tomatoes offer a subtle sweetness that helps to balance the overall flavor profile. You can add tomato paste, diced tomatoes, or even a splash of tomato juice to your gravy. Start with a small amount and taste as you go, adding more until you reach the desired balance. Simmering the gravy after adding the tomatoes allows the flavors to meld together and further reduce the salty taste. The longer it simmers, the more the tomato flavor will integrate into the gravy.
If you're using canned tomatoes, opt for the 'no salt added' variety to avoid inadvertently increasing the sodium content of your gravy.