If you want to move your landline to GV, there is a procedure, but it is not as simple as expected (for technical reasons). There are discussions here on how to see if your number will port and how. You never cancel a number or close an account with an existing company before the port is complete to the new provider and tested.
If you just want to use an existing GV account and number, then any of the Obi Adapters will work for that (assuming your phone is a touchtone phone). I would personally set it up and test it first before cancelling my landline.
The Obi acts like a phone company, connecting your telephone to the provider (GV in your case) through the Internet.
Although GV is great because it is free, it is not without a few small problems. At some point you may wish to get a voip provider who offers a few more options than GV offers. To get started, GV is a neat thing to use.
If you wish to have multiple phones in the house all connect to the Obi as is common with the old phone company, that is possible with existing phone lines inside the house. HOWEVER... you must ensure that the lines to the phone company have been disconnected or else you can damage your Obi. There are instructions else where here in the forum on how to do that and probably Youtube videos on the net.
As you may know, that GV does not offer 911 service. You could add another voip provider who does and have 911 calls sent through them. That is the neat thing about multiple carriers; you can pick and choose who you want to handle specific calls, etc. For example, one carrier may be cheaper for overseas calls than GV.