Suppose you set a training session consisting of those: Warm Up, Shooting, Set-Pieces, Pass-Go-Shoot, Stretch. This is an example only; it may or may not be a decent training session.
For shooting session, you need appropriate players to shoot, this could be STs or AMs, but you also need a GK between the posts.
For a set-piece drill, suppose the defending one, you need the attackers (usually those that can head the ball) in addition to the defenders (DCs and GK).
For a Pass-Go-Shoot drill, you need a GK to make the saves in addition to players for passing, movement and shooting.
But, do you need others team players to be involved in a GK training session only, for instance, Warm Up, Skill Drill, Carioca With Ladders, Sprint, GK-only drill, Stretch? No, as it involves a GK-only drill that outfield players don't require to be involved in; if they are involved, this will waste their condition for non-related gains or for no gains at all for performing the specific GK-drill.
This is up to the user to find out all this, but I have made an exception here; I have showed you some of it.