I think it's a fair question, actually, and it raises some key points:
1) There's a big gap between the two schools when it comes to how many top recruits each has. It's glaring when a high recruit up north apparently doesn't live up to his potential, because he's supposed to be THE guy. At OSU, there are so many top recruits that it's easy for us to focus on the hits and not the misses.
2) Along with that, there are so many top recruits that it's probably not even possible for all of them to succeed. Gill's been sitting behind guys who are getting drafted into the NFL and having success there. It's hard to get the reps, hard to get the attention.
3) It's also hard to say whether Gill has developed or not. Joe Burrow transferred to LSU, and he continued to develop there, but does anyone really think that he didn't grow as a player at OSU? He was behind another really good QB. If Brian Hartline is as good as we all think he is, then it's pretty likely that Gill leaves here a better player than he came in.
4) Maybe the Michigan sample size is just too small, but the numbers certainly back up that OSU is consistently developing players well. I've seen the data posted elsewhere here about how our 5*, 4*, and even 3* players do in terms of making it to the NFL. Every school is going to miss on players, no one here is claiming that OSU is 100% when it comes to getting the most out of every player; maybe Michigan is going to have a string of players that they develop into total studs, but at this point we haven't seen it, and seeing is believing.