Making Gold part of GamePass is a terrible move for me personally as I have zero interest in the library mainly because I have a huge one of my own.
It would also be a huge blow to the digital sales on the Marketplace and will probably piss some publishers off.
I think there is a stronger chance that MS will be making it free yo everyone,it will be a huge selling point for their box and will also aligns well with Phil's strategy of breaking down walls (cross play,play anywhere,play everywhere with xCloud).
Also,it just seems weird that their flagship device is the only one where online multiplayer is behind paywall (PC is free and multiplayer with xCloud is on by default).