GH measures the hardness of the water, whereas TDS measures the total dissolved solids in the water.
GH measurement gives you the approximate content of Calcium & Magnesium.
While TDS gives you the approximate content of Calcium & Magnesium, as well as other dissolved minerals, metals, organics, etc in the water. So apart from Calcium and magnesium, what are some of these dissolved solids that the TDS meter is picking up?
It will pick up minerals like Sodium, Potassium, nitrogen, phosphorous, sulphur, silicates as well as dissolved metals like Iron, Copper, Lead. It will also pick up Nitrates, and chemicals and pesticides that might be in the water, just to name a few.
So as you can see, GH measurements can reflect TDS measurements if all you have is Calcium and Magnesium. But reality is that you will have more than those minerals in your tank water. So the TDS measurement will usually be higher than the GH measurement, since the TDS meter is made to detect more.