GH adjustments is hardening or softening your water.
Chemical GH adjustment
Chemical GH adjustment products usually soften water by using sodium ions to exchange with calcium and magnesium ions. (Calcium and magnesium ions dissolved in water is what gives water hardness value). Technically this softens the water but practically fish dont feel any different.
Natural GH adjustment
Dried leaves, driftwood etc
These materials release tannins and other humic acids into the water. There are many ion-exchange where calcium ions gets exchanged for H+. The H+ released into the water makes humic substances "acid."
A favourite is ketapang leaves, however you may or not be able to find it in your area.
You can use peat moss to lower water hardness (ie soften water). Put the peat into a filter bag so it doesn't dirty the water. The bag can be put in your tank or in your filtration system.
To accelerate the process, you can boil it like the way you make tea and use the solution by itself.
RO Water (reverse osmosis water) or distilled water
Having no hardness, these pure water can be mixed with your tank water so that the concentration of magnesium ions/calcium ions in your water is spreaded out over a larger volume, thereby diluting the hardness and softening the water.