Correct, TDS is a total of all things dissolved into the water. IE, fertilizers, trace minerals and salts. That includes your GH and KH. If you were to test just GH and KH, they're 17.9 ppm per degree. So 179 GH and 0 KH would be 10 GH
So if you know your GH and KH, just multiply by 17.9.
So if you know your GH is 10, and KH is 0, and your TDS is 250, you know that only 180 TDS of that is GH, the other 70 TDS is other things in your water such as nitrates, potassium, phosphate, etc.