Gameplay gates are conditions that the game state must meet in order for the player to able to access and "play" the moment.
The current value of the progression element specified must be more or equal to the "Min." value and less than or equal to the "Max." value.
The character '~' means that there is no enforced value. e.g. in the example above, all the player needs to pass the gate is to have one "Room key"; they can have as many of them as they like.
As well as static values for minimum and maximum quantities you can also use the current state of progression elements. For example:
This allows you to do interesting things similar to RPG-style skill checks or use progression elements as enums (see here for an example).
This will subtract the value of the gate from the progression state when the moment is played in Walkthrough mode. e.g. if a moment is gated by a subtractive gate of 1 Gold, 1 Gold will be subtracted when the moment is played.
Set the gate to subtractive by checking the "Advanced gates" checkbox, then the "Sub." checkbox.