From price to power consumption to available tracking channels to, well, just about everything, the PhotoTracker Plus is a step ahead of the GP-1 when it comes to geotagging your photos on a Nikon DSLR.
The GP-1, if you recall, is Nikon's first party geotagging dongle. You attached it to the hotshoe and away you go, tagging photos with GPS data. But therein lies the first differentiating factor: the hotshoe. The GP-1 uses it, while the PhotoTracker Plus does not. In the latter case, the 10-pin terminal on the front of the camera body is used, freeing up the hotshoe for a flash.
Next is price. The GP-1 is approximately $200 today, while the PhotoTracker is $179. It's a slight difference, but it's there.
Then there's the aforementioned power consumption and tracking channels. Nikon's dongle sucks up about four times more than Gisteq's dongle and uses just 18 channels, while the PhotoTracker uses 44.