I think this is overblown and based on a simplification of what an OC does based on what the fans see (calling plays on game day), which is really a small part of the job. Peyton might audible to a different play to beat a Cover 2 look or something, but he's not designing plays, running drills in practice, creating the game plan, correcting fine points of blocking technique, etc. - everything the OC does the other six days a week. What the coaching staff does on game day pales in comparison to what they do the rest of the time to get ready for game day, and as far as I know Manning isn't much more involved in that department than other quarterbacks.
As for what Gase brings in particular, MMQB recently had a great article with Mike Martz
breaking down some Gase-designed plays. He seems to have a strong ability to identify how defenses react to different looks and how to exploit them. Whether that will make him a good head coach or not, I have no idea.