GH measures Calcium and Magnesium plus some others things. You only need to worry about false GH readings if adding Iron fertiliser. GH alone does not influence pH.
KH measures carbonates and is independent from GH, but but but, most tap water contains Lime which is Calcium carbonate so you tend to get GH and KH in parallel. It's possible to have high GH and low KH eg a solution of Epsom salts aka Magnesium sulphate, or the opposite eg a solution of Sodium bicarbonate.KH directly influences pH.
TDS literally means Total Dissolved solids so it measures everything in solution. That includes fish waste, fertilizer, dechlorinator, tannins, common salt etc etc etc. A solution of Common salt has high TDS but zero GH and KH for example.
pH is the measure of how acidic or alkaline a solution is, at a molecular level this means the concentration of hydrogen ions (H+).It's not as important as once thought and it seems that TDS is more important to fish, however, in nature low TDS tends to mean low pH (& GH + KH) and vice versa and generally speaking if you get the TDS correct for your fish the pH will be in the correct range.
To complicate matters KH is involved with the chemical reactions that occur during the nitrogen cycle. So as ammonia is metabolised by bacteria so are carbonates. This leads to a drop in KH and pH over time. If the KH is low to start with or not replenished the filter bacteria stop working. At the same time the pH crashes and ammonia builds up. Some people call this "Old Tank Syndrome".
Most aquarists won't notice this because, as luck would have it, at low pH ammonia becomes ammonium which is less toxic. However, if the aquarist decides to do a large water change, the pH may rise back above 7 and the ammonia becomes toxic again. The fish become very sick, very quickly and may even die.
So, you bought an RO Unit and you don't know what your doing?
Pure RO has practically no KH, GH or TDS. It has a theoretical pH of 7 but any small impurity will swing the result, even CO2 from the air.Clearly this is inappropriate for most fish, so how should you prepare it?
Firstly, it takes some time to make so you might as well add an air stone to the storage container as RO even removes oxygen. You may even choose to add a heater but that is optional.
Finally, you need to add buffers or minerals to match the requirements of your chosen fish. To do this you must research the fish you have and find the optimum conditions for keeping them.
Luckily you can purchase commercial RO minerals (or make your own) that do a very good job and simply adjust the TDS to what you need. The GH, KH and pH will fall roughly in range.
However if you need to raise GH or KH a little more this can be done by adding Calcium sulphate or chloride for GH or Sodium or Potassium (bi)carbonate for KH. Some plant keepers prefer the latter as Potassium is an important element for plants.
Adjusting pH is not usually necessary but raising KH will raise pH so that's easy but reducing pH requires some care. If TDS/GH/KH are high then adding acids is pointless as you only achieve hard, acidic water which is completely unnatural. But if you have soft water and want to drop the pH a little then commercial "pH down" products can be used but organic acids from Alder cones, leaves or bogwood are less hazardous to handle and won't cause sudden pH drops. Always monitor pH and KH closely if using anything to lower pH, not forgetting that low pH/KH can cause filtration to fail.