Gomez was left out because he will be hoping to play against Germany on Friday. He has another 41+ chances to start or be sub in league matches, May never get another chance to play against Germany. A good performance there would raise his profile and value immensely.
As Eddie says, it will increase his value, plus it will give him the experience of playing 90 minutes of football, which he won't get at Charlton.
You are thinking as a Charlton fan. Try thinking as Gomez. We don't want another Poyet debacle. Gomez would be welcomed by any club in the Championship, and many in the Premiership. If he or his advisors think we are hindering his development he will be off like a flash, almost certainly to earn a lot more than we are paying him. Although you are supporting my point. I suspect he will only be a sub there too. He is giving away a year there as an u-18 playing for the u-19s. But at least he will get his foot in the door, with a chance to impress at training, etc. The other reason Fox was chosen for the bench instead of Gomez is that he is the best replacement if Wiggins was injured, while Wilson is excellent cover for Solly, as is Morrison for ben Haim and Bikey.