I have used GS training quite extensively and have definitely noticed a drop in its effectiveness. Last season I had better GS updates when I trained stamina even when my players had 90 minutes. As well, when I didn't training GS, it wasn't uncommon for my players to pop to 9 GS when they had 80+ minutes.
GS updates are probabilistic. No number of minutes will guarantee your players pop or stay at 9. Getting a good range of minutes simply makes it more likely for your players to improve or maintain good GS.