how are you setting your variables? is the blueprint that your showing one that is placed in the scene? what is this game master blueprint that your mentioning? is it one you made or is it something like the game mode bp?
the setup of this blueprint will determine how to get the references. for example if you place this bp in the scene then you could use a public variable to set the values. if this bp is a game mode then you could easily use the level bp to get the reference then set a values in the game mode. but it all depends on how you have things set up now.