The GPU file is not used now, it's jut a stub that we might use in the future...
The ladder will be used automatically without any need to set any condition, if the marshaller distance which is calculated starting from the eyepoint position of your airplane is too high, and will likely result with the marshaller placed inside a building.
Note that this might not be noticeable on a parking in the open (without a jetway), which will allow the marshaller to be placed farther away, before deciding to use a ladder.