The Speed : Over two seasons (approx. 28 weeks of training), an 18-year-old training 1-position OD at 0.6 pops/week will get roughly 16.8 pops. With a Gym (10% boost), they get 18.48 pops.
Except that's not how gym works. With a level 3 gym, you get 10% of the training (so 0.06 in your example) randomly attributed to one of the 12 skills, and this happens three times (i.e. the level of the gym). So after two seasons, the expected gain per skill is approximately 0.4. If your trainees have low sublevels, it would not be impossible to observe no pops due to the gym even after two seasons.
The Price Cliff: In BuzzerBeater, the market isn't linear. The price difference between a player with level 14 (Wondrous) and level 16 (Colossal) OD isn't $200k—it’s often $1M to $2M.
Did you mean between level 14 and 19 (which is colossal)? Because if you really believe there is a $1M to $2M difference between a level 14 and 16 in OD, that is once again a proof that you have no clue about the transfer market.
Asset Multiplier: If you are training 6 players (as you suggested), that 1.68 extra "pop" applies to all six players. Even if it only pushes 3 of them into a higher skill tier, you've made an extra $2M+ in sales. This easily covers the $1.42M investment (Gym cost + 2 seasons of upkeep)
But there is absolutely no way you get +$2M from cross pops in your example. I really have the impression you are making up numbers in order to avoid admitting you were wrong about the value of the gym.
I would love if you could prove me wrong by providing some examples from the transfer market though, but I think this will be be difficult to do.