To my understanding you must be at least obese (in order to walk into the stable and be heavy enough to trigger the pressure-plate) after that you must drink all the milk yourself. (it's been a bit since I've done this ending so I may be wrong)
This is correct. Spoilers, obviously, but https://pastebin.com/viYCgj7q has a full list of all endings and how to get them (please let me know if any are not working).
According to the list, you can pick the gum without starting the demonstration. However, there is no prompt to place it in your inventory after talking to Wonka.
Is this a bug or was it removed from the game
Edit: found it!
You need to ask Wonka for a demonstration and then a prompt will show to take the gum.