you need to look at the "type of pop" because as Alonso also mentioned that, you can either have:
- a pop from cross training which has nothing to do with Gym (one example is, OD is also gives a little bit to HA and DR and middle amount to ID)
- a pop from gym`s effect which is at level 3 a random 10% skill (so it is like if we stick with the OD and you also got a pop in RB)
In overall I would say it is worth the investment if you are maximizing the training and this is an important thing for you.
If you are under pressure like U21 and you "only have 4 seasons" to get the best possible out from your people it matters.
If you are a type of person who like to maximize the effect irrespectively from U21 or NT still worth it.
If you are just training in an ad hoc type basis, because you got an MVP draftee and you think ahhh I got an MVP, I got an MVP I must train him now, I do not think in this it is worth it.
Sz.