You have to train them now too nothing changed. A freshly caught level 20 tentacool with the reusable Tm move scald still has to be trained just as much as a level 20 tentacool with the HM surf to take on a fire gym. In fact surf has a higher base power so you could say technically the scald tentacool has to be trained more than the surf one....But at least then you had to train the fish or birds before you took them to the gym