I have been having the same problem. Seems to be wear somewhere. In my case I found that bending the rubber port cover on the camera and tucking it tight against the plug from the GP-1 keeps it online. Not a long term solution as I'm concerned that the port cover will either distort or come off. I'm going to try gently widening the plug end to try to get a tighter fit. I've had mine for about 18 months and seem to remember it being a pretty tight fit in the early days. Tried blowing out the plug and socket with 'tinned air' but that didn't help.
If anyone has a solution, please post it hear. I wonder how many of us have this problem and whether it is D7000 superficial?