If you purchased Salty Shrimp GH+, that is a premixed mineraliser. That means the manufacturer has premixed Calcium sulphate and Magnesium sulphate together at a ratio of around 4:1. If you want more control over the Ca:Mg ratio or if you want to make GH/KH+ you can purchase these ingredients and mix your own.
I have a sticky post in Water Parameters, if you are ever curious of making your own remineraliser.
If you want a bit more control over the parameters you get from Salty Shrimp ... mix it into a plastic water bottle. Fill 1/4 or 1/3 of the bottle with SS GH+ and add RO water. Dissolve the SS GH+ and drip only as much as you need from the bottle for each water change.
I will try and answer this as best I can............
The commercial GH+ or GH/KH+ powder is made to create a set level of parameters and minerals that suit the shrimps requirements perfectly, to help them stay healthy and moult etc! Using RO water it will produce a certain preset ratio of TDS/GH/KH/PH, dependant on which product you use.
Think of it as if making a cake, you need certain ingredients and the right proportions of each ingredient for it to work well, too much (or too little) of something means it won't work properly?
Some people prefer to make their own mixture of minerals rather than buy the commercial products!
Hope this helps you understand?