Just want to repeat that this thread is huge but has many interest postS so it's worthy of reading it specially if you are ambitious player and want to go deeper in the game.
So, saying again about GK training method - you can't save greens from this or improve better a player but it saving time and have better control of the skills you want to add in power-training.
For example, I bought six new players , in two of them I want to give sa and position, two are defenders and two are attackers.
I put all those six together to make some sets of GK drills.
Two players are gaining from this some progress from gaining the sa and position.
Then I take the two defenders and put them one drill of "Press the play" (they gaining the massive relative skills from this) and then the two attackers and put them one "Wing play" drill.
If I wanted to do that with the normal way, I would spend double or triple time.
Another case is if I want to put the skills Speed-Passing-Shooting.
Those are in "Pass, Go and Shoot" drill but this drill is easy and has -1,5 %.
Using many GK sets and put one final drill of above, also saves me from a lot of time.