Apologies for late reply. I helped in incorporating the GP3 into the ioHub libraries a couple of years ago now. As for how stable the code is, it is as stable as anything else that has been incorporated into ioHub. The developer of ioHub, Sol Simpson, helped me with the coding. However there are few points you need to know.
Firstly the code will work with original GP3 eyetracker, Gazepoint have developed a new GP3 HD eyetracker, I have no idea if the code will work with new device.
You will absolutely need a computer that has a minimum spec of at least an intel i5 processor.
The GP3 has a sampling speed of 60 Hz, and claims to be accurate to 0.5 - 1 degree of visual angle. I found this to be the case only under absolute optimal conditions.
The calibration is not incorporated into ioHub, so you will have to use the app that comes with the GP3 to calibrate it first, then go into psychopy.
Hope this helps.