Guys. FT is not a switch that can be strictly 1 or 0. There is a hidden parameter that multiply the incoming XP, I call it Talent. We just agree to call FTs those players who have this parameter high.
There is no border between FT and non-FT except the one in our mind.
GKT adds XP exactly as any other drill, but other drills add this XP to certain skills while GKT don't have skills to improve, so XP is left undistributed until you give the player any other dill. Any other that improves skills. In that moment undistributed XP is going to those skills.
Stop casting your voodoo spells, GKT is not a magic nor exploit. It is not obligatory for anything. It is useful only if you want to save some time during power-training or if you have GKT of higher level than the following drill (because it will give level bonus to undistributed XP of 10, 20 or 30% for, respectively, lvl 2, 3 or 4 of GKT).