You could expose such subsystem to blueprints if you first create a new class in c++ and make that “Blueprintable”, then inherit from the new class.
.h file:
#pragma once
#include "CoreMinimal.h"
#include "Subsystems/GameInstanceSubsystem.h"
#include "MySubsystem.generated.h"
UCLASS(Blueprintable)
class MYPLUGIN_API UMySubsystem : public UGameInstanceSubsystem
{
GENERATED_BODY()
private:
protected:
public:
};