GBTimelapse doesn't support Nikon cameras at this time because of several problems with Nikon DSLRs.
To capture high quality time-lapse, you need to minimize flicker (changes in brightness) between successive images. This requires operating the camera in bulb mode.
1) Although Nikon provides an SDK (Software Development Kit) for their cameras, it is incapable of controlling the shutter in bulb mode. The Nikon drivers only have a command to "capture" an image. The driver provides no ability to hold the shutter open. It would be possible to use a third-party USB controlled shutter cable to operate in bulb-mode, but this adds expense and complexity.
2) Nikon cameras have a peculiar way of implementing bulb exposures. It is not possible for the shutter to open for an arbitrary length of time. Instead the shutter opens for a multiple of a short duration (on the order of 0.020 seconds). This prevents the bulb time from ramping smoothly (in 0.001 second steps) when the light is changing at sunset or sunrise.
3) Nikon lenses cannot perform the "lens-twist" technique used by Canon lenses to eliminate aperture flicker.
These are the reasons we have not yet implemented a Nikon version of GBTimelapse. Plus the fact, we would need to purchase a slew of Nikon gear to get this working.
Given the above considerations, we have found a way to implement bulb control with Nikon that requires an extra hardware device. The device would require an extra expense on your part. We need to know if it is worth our time and trouble to go ahead with this plan. Would Nikon owners be willing to spend the extra money to purchase such a device? It would eliminate the bulb control problem, but bulb resolution and bulb flicker would remain. These problems could be minimized in post using GBDeflicker.
So here's a 'survey' question for you...
How much extra would you be willing to spend? $100? $200 $300. Please submit a request with your opinion.