Commute from JP is doable, but as with any commute, you lose time at home vs. just living in base. If he is currently commuting from JP to the US, I can’t imagine GUM would be any worse. GUM RSV rules allow you to “compress” all your off days into a single block. It is possible to do basically the same with a line based on your seniority & as long as you are willing to sacrifice trip quality for scheduling preferences. Some guys even use this to commute from the mainland US, although I wouldn’t personally put myself through that.