GK training is just a "spring" you push and let it go after. Nothing "magic".
All the other factors about training remain the same
- A young player 18-21 y.o. will have much more gaining than an olders
- Not all the young players are fast trainers. There are classes
- The quality of the player matters (a 4*-5* gaining more than a 5*-6* who is gaining more than a 6*-7*)
- The level of the drill affects
- The "white" color skills gaining more

Where I find useful the GK training trick is for a massive power training.
If you have for example 7-8 players to train and some 200-300 greens to spend, you can set them in GK drills all together and then take the defenders and give them one "Press the play", the wingers "Wing play" and MC or ST what else you want.