Google Voice numbers are "locked" by default and must be "unlocked" prior to submitting a port request to move them to another provider. Google provides documentation on how to unlock your number with them here: Google Voice Support. They currently charge a $3 fee to do so.
Once the number is unlocked, a normal port request can be submitted as outlined in Porting Telephone Numbers to Primecall.
When submitting the completed LOA by emailing support@primecall.com, please include a copy of the unlocking receipt obtained previously as well as a screenshot of the billing page from Google that shows your name and/or company name with address. This will help ensure that the process is completed as quickly as possible.
Once your phone number has successfully ported, it's always best practice to notify the losing carrier (Google Voice) that you've ported your number out.
In addition to notifying the losing carrier of the port-out, you should always request that they remove your number from their system on port-out. This helps prevent any call translation issues from occurring, such as other customers of the losing carrier not being able to call you at your new service provider.