Interestingly enough, the main point of contention for moving Gallon to RB, isn't, " we have better options at RB", rather, "Gallon is our best receiver".
If Fitz is out of the game, which RB on the team would be a better option than Gallon? I don't think there is one at this point.
All things being equal, from what I've seen this season, if Fitz had to miss a game, and M had a week to get there best overall RB ready, Gallon would be the best choice. Best runner, best receiver, best overall.