You are very welcome, my friend, I'm glad you find it useful, but that's not what I meant. If you have Skill Drill on lvl 4 (world class, +30%), there is no need to use GK Training before it. The efficiency of the training (XP gain / condition loss) will be the same. You are just making the training a little bit quicker for yourself, but it's not really important. Just saves you a few clicks, nothing more.
The point is you can use GK Training before any drill with lower level to "enhance" it.
Let's say I have Wind Play lvl 2 (semi-pro, +10%; 3% CL) and GK Training lvl 4 (world class, +30%; 3% CL as well). In this case each Wing Play gives 4 x 1.1 = 4.4 XP to a player and GKT gives 4 x 1.3 = 5.2 XP.
Therefore, 6x WingPlay give: 6 x 4.4 = 26.4 XP
5x GKT + Wing Play give: 5 x 5.2 + 4.4 = 30.4 XP
It's 26.4 vs 30.4 with the same condition loss. Efficiency of the second training is higher, you gain more for the same price.
The difference becomes even better in case you use several "full GKT" trainings, like this:
6x GK Training
6x GK Training
...
5x GK Training + [any lower lvl drill]
Just don't end your training with GKT, because experience from it dissapears before a game if left undistributed. Well, at least in the new version.
Obviously, I'm only talking about field players here, not goalkeepers.