Does GS has any connection to age?
for myself, i think i have found
(a) that the minutes guide changes over the course of the season -- i think more minutes are considered beneficial early in the season, detrimental later in the season.
(b) that getting players with high stamina to the highest level in GS is difficult and twitchy
(c) GS training is very powerful.
i hope Ozone answers because he has mastered GS -- by this time of the season, it is the rare player on his squad whose GS is not proficient.