Yeah, we forgot to get to that one :p
Because Mentor is so much better than the next best substitute (Pyromancer or whatever), restricting Mentor, I believe, would dent Gush's overall prevalence, but it's hard to know by how much. Could be as little as 5-10% or could be more on the order of 25-33, or even 40%. That's much harder for me to predict. But that's probably the range.
If Gush is restricted, I think Mentor will end up being restricted when it's discovered that restricting Gush had no suppressing effect on Mentor's prevalence.
When I built and tested Nahiri Control and Saheeli Oath (both decks built around the Gush draw engine), the worst matchup for me by far was Gush Mentor - they actually did very well against the rest of the metagame. You are right that if Gush is restricted Mentor will probably need to be restricted. The problem is that the converse is also true. If Mentor is restricted and not Gush, that doesn't make the other Blue draw spells better than a bunch of cantrips, Gushes, and Delve spells. That same section of the metagame moves to Walkers, Oath, Young Pyromancer, or even Managorger Hydra to close out games after drawing a million cards - win conditions that are arguably better against Shops and the rest of the field (certainly more diversified and harder to hate out with Walking Ballistas). They don't start playing other Blue decks because the gap between Gush and the other draw engines is as large or larger than the gap between Mentor and the other win conditions.
Where did this false dichotomy come from that either Mentor or Gush is the problem? That either Mentor or Gush need the be restricted? They are both huge constraints on the Vintage metagame and it is reasonable to restrict both of them.