Hello, and welcome to the forum.
To access the GameInstance which you configure inside the Project Maps&Modes panel, you call the GameInstance usually on Event BeginPlay and also set a reference for it.
A child BP / Blueprint can access the GameInstance by calling it, i.e. Cast to MyGameInstance then get VariableX value.
The GameInstance is usually not a child BP and you just create one for your project. The GameInstance can be accessed by all Players or Actors, for instance to get a variable about the current time of the day.
To save data for later use (on game restart) you need to create a savegame and setup a structure inside the SaveGame BP to store your custom values. To temp save data you can store it in the GameMode, which is assigned to the current Map (and lasts for the Map) or you store it on your Player Controller for instance.
There are good written posts and probably videos too on the basics of GameInstances (Lasts entire Game Session) & GameMode (Lasts duration of current Map), or for handling SaveGames (permanent stored on user computer).