Google Voice is an inbound telephone call forwarding system that works over the Public Switched Telephone Network (PSTN), forwarding calls to the forwarding phones you define on your account. The call quality of inbound calls forwarded to, and answered on any sort of telephone (land line, mobile or VoIP) will vary and depend on the quality of the complete path over the various phone carriers involved in the call.
Outbound calls using Google Voice on a mobile phone do use the mobile phone’s voice service, whatever it may be (GSM, CDMA or LTE). The app first uses your phone’s native voice service to call a Google Voice gateway, then the gateway places your call to the called party, and bridges the call together. So, again, call quality depends on the underlying carrier services involved in that path.
Hangouts phone calls travel over VoIP, using the G.711u PCM codec, and will have call quality equivalent to the best land line connections. G.711u is a lossless codec, and uses approximately 90Kbps per direction (64Kbps for the encoded audio, plus some overhead for the IP traffic).
Outbound calls using Hangouts on an Android or iPhone use whatever data connection the phone is using (either your carrier’s data connection or WiFi). Again, call quality will be entirely dependent on the quality and reliability of that connection.