This past Jan I was on CO to NRT transferring to NW onwards to SIN in BF. I had no problem whatsoever in leaving my luggage/coat/packages in the reciprocol lounge in both directions. Some may say, since the bins don't lock you are taking a risk-- but I've (knock wood) never had a problem. I never leave my cash, passport, tickets etc in my 'left' luggage. I also don't 'ask' if I can do this, so as not to get a 'no' response. I just check in at the main desk, show my BF ticket, find the storage area, leave my stuff and depart. They don't know if I'll be gone 5 min or 5 hours!! In NRT there are a couple of lounges available, choose the one closest to your ongoing flight to GUM to store your luggage.
That way you won't have far to go and can spend more time in the city. I assume you'll be taking the train in?? It is very simple to find, and the information desk on the lower level near the trains is very helpful.