It is possible to use GK Training before any drill with lower level to "enhance" it. I'm talking here about field players only.
Let's say I have Wing Play lvl 2 (semi-pro, +10%; 3% CL) and GK Training lvl 4 (world class, +30%; 3% CL as well). 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 Wing Play 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.
I've verified this with 1-on-1 Finishing level 1 and GK Training level 3 (+20% XP).
I trained a striker with two following sessions, 60 times each:
1) 5x 1-on-1 Finishing. 7.5% Condition loss, 5 * 2 = 10 XP
2) GKT, GKT, 1-on-1 Finishing. 7.5% Condition loss, 4 * 1.2 + 4 * 1.2 + 2 = 11.6 XP
So the second one should give 16% more experience that first one.
Here are the results:
Column 1: total gain from 60 sessions with 5x 1-on-1 Finishing
Column 2: total gain from 60 sessions with 2x GKT and 1x 1-on-1 Finishing
Column 3: amount of experience gained from the second training schedule compared to the first training schedule; 116% expected.
I would be thrilled to hear an official answer: is it an exploit and should be fixed or a feature and I can use it?