Chaos Gym is in my cube and gets drafted quite frequently. Of course when the card was printed, Supporter cards didn't exist, but as a pre-Diamond & Pearl card the 'Trainer' reference should include Trainer--Supporters. The verbiage on Supporters has consistently been that you "may play only one Supporter each turn". However Chaos Gym states that on tails, "the player can't play that card." That leads me to believe that since the card could not be played, it wasn't considered played in the game state. (Some effects may happen anyway due the effect being used, such as if the card was Misty's Duel or N.) Therefore, the player could attempt to play another Supporter in the same turn, as Chaos Gym making it fail did not "use up" their Supporter for turn. Side note: Pre-BW Supporters were 'put in play' by your Active and so wouldn't be eligible for opponent's use, but since my cube includes BW-on supporters we treat them the same as modern supporters to keep things lined up. Irrelevant to the question at hand, though. Asking here since I doubt there would be any "official" ruling on an old card interaction. The closest I found in Compendium was in regard to the old Gym Challenge Blaine Trainer card that allowed extra energy attachment. -Kooper, who omitted Neo Genesis Slowking from the cube, but is pretty sure failing a Supporter due to Mind Games would use up your Supporter for turn.