The Gh is the hardness of the water which you dont need to play with its the KH that is the important thing with fish. The KH will help keep the PH to be stable. So I would check your KH and see what the level of that is. Now moving the KH can adjust the PH but in slow bits.
The GH only needs to be changed if the fish need hard or soft water or the plants need it. Goldfish in general will adapt with hard or soft water