GH is good to know, but you don't really need to test it on a regular basis.
The GH test kit works fine, but can be finicky, especially if the reagent is older.
Some people also find the colour changes (it is a titration test kit) a bit hard to discern.
I wouldn't worry too much about pH. A stable pH is more important than anything. If you are looking for a lower pH for a particular reason (i.e. such as breeding or keeping wild caught fish), then rather than using bottled water (which would become very expensive), RO or DI water would be a cheaper alternative.