I no longer need a virtual PBX, how can I port my numbers with RingCentral to another service like GoogleVoice?
Porting is always handled by the new provider. Google Voice is a little strange in this regard. As I understand it, they only allow you to port in mobile numbers. So you would have to first choose a mobile provider to port to and then port to GV. I would choose a cheap prepay mobile provider to port to first and then login to GV and follow their instructions for porting your mobile number in. Good luck.
Here are some more detailed instructions: https://www.cnet.com/how-to/how-to-port-your-landline-number-to-google-voice/ and a page you can check if Google will allow you to port in: https://www.google.com/voice/b/0/porting