UserId = FirebaseAuth.getInstance().getCurrentUser().getUid();
final DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference gameInfoRef = rootRef.child("user").child(UserId).child("gameInfo");

gameInfoRef.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {

        ListView lv_SelectGame = findViewById(R.id.lv_SelectGame);
        GameList.clear();

        for (DataSnapshot ds : dataSnapshot.getChildren()) {
            Game game = ds.getValue(Game.class);
            GameList.add(game);
        }

        History_SelectGameAdapter adapter = new History_SelectGameAdapter(ida_History_SelectGame.this, GameList);
        lv_SelectGame.setAdapter(adapter);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        // Getting Post failed, log a message
    }
});

gameInfo
    -LMIdDTDiUd3ajVVMdzC
          gameDate: "2018-09-01"
          gameMode: "SomeMode"

    -LMOGEUbCTsaeywPICqi
          gameDate: "2018-09-06"
          gameMode: "SomeMode"

    -LMQfCQ7TO21WP3qW7v3
          gameDate: "2018-09-15"
          gameMode: "SomeMode"

lv_SelectGame.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, final int position, long id) {
            ListView lv_SelectGame = findViewById(R.id.lv_SelectGame);

            Toast.makeText(ida_History_SelectGame.this, "getKey() of this item", Toast.LENGTH_SHORT).show();

        }

lv_SelectGame.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, final int position, long id) {
            Game game = GameList.get(position);
            String key = game.getKey();
            Toast.makeText(ida_History_SelectGame.this, "getKey() of this item", Toast.LENGTH_SHORT).show();

        }