When using GBTimelapse AutoRamp in bulb mode with neutral density filters, it's best to know the exact value of each filter. Even though a filter may be rated at 9 stops, it may not be exactly 9 stops.
I have found filters that are more than ten percent different from their rating.
To avoid a change in image brightness when adding or removing a filter, AutoRamp needs to know the exact number of stops of the filter.
There is an easy way to calibrate a filter using AutoRamp itself. This video shows you how.