Did Missy develop the Cyber-rain by studying the Fortold and using trans-dimensional technology to help convert the dead into Cyber-men?

Gus's monocle did look like 3W, and the tech that kept the Fortold alive and fighting in a zombie-like state could have contributed to creating an obedient Cyber-army.

I think its an interesting idea anyway.

I think it probably was Missy, yes - whoever was controlling Gus knew how to ring the TARDIS phone. I doubt her Foretold findings had much to do with her Cybermen plot, but I imagine we'll find out more next year.

The why's and wherefore's of who's behind Gus was not explored (at least on screen). Deliberately so, I guess.

And, I agree, the obvious suspect is Missy, but that doesn't mean it will turn out to be right.