@Smmenen Gush and Mentor could both easily survive on their own. However, they both lose some power if the other is restricted. As the best way to play them is to play them together. If we are playing on the conservative side of restriction only one should go at a time.
I'm not sure that Mentor decks would automatically get larger with a Gush restriction. The faster Mentor combo decks exist to beat you before Gush is relevant. I think there is a distinct possibility that these decks slow down to beat Shops and Eldrazi again. Maybe the slow blue list is something like Mentor Standstill type, but I don't think that's a guarantee.